Habitat Entrance

The moonpool of habitat Hydrolab
Moonpool of Hydrolab

(Updated 29.11.2020; re-edit of the hatches chapter) Surely one of the most complicated area of an underwater structure for human occupation is the habitat entrance. It is the local water-air interface, vulnerable to changes in pressure by tidal movements on the surface of the sea; its hatches have to bare potential pressure differences between the habitat interior and the surrounding water; no object that is only a little bigger than the greatest diameter of the entrance can be brought into the habitat. It is constantly wet and humid and the only gateway for medical assistance. But to see in detail let’s have a look at the different sections of the entrance complex.

The Trunk

The trunk is the skirt under the entrance hatch and it is generally holding a certain amount of air. Even if the hatch above is closed it can serve as an emergency air pocket allowing divers to emerge partially or completely from the water. There they are able to talk face-to-face, solving hatch problems in a calm way or just save themselves from any emergency. Above the trunk is the hatch, that can be opened up to the inside of the habitat, down to the trunk itself or (with two hatches) to each side. The first trunks were small and simple, but several projects showed that they should at least have the size for two divers and seats just below the water allowing returning divers to sit and take off their diving gear. Therefore they should have well lighting, handles and a system for fresh air supply.

Some habitats had scuba tank refilling installations inside the trunk. This avoids lifting of heavy equipment; it simplifies taking equipment on and off; and it cools the tanks while being refilled.

The Underwater Access Space

The underwater access space is located below the trunk between the seafloor and the trunk entrance. It requires enough space to easily enter the trunk. Therefore it determines the height of the entire habitat over the sea bottom. Several projects like Tektite or Conshelf had a “shark cage” at the beginning of this area preventing uninvited animals from approaching the entrance. The access of this area should be very well marked (e.g. by position lights) to guide divers in bad visibility.

Access corridor through balalst section; on top hatch to habitat interior
access corridor through ballast section (pink) below hab interior

Features of this configuration:

  • completely flooded, preventing influences on general stability
  • full lit up when outer gates are closed; providing time for procedures before leaving the habitat into the sea or after returning
  • red lights for night dives while outer gates are open to prevent attraction of fish blocking the trunk
  • appr. 220cm high to maintain enough space for moving in lower underwater gravity
  • floor grate to let sediment settle down without covering the floor. These grates are removable to vacate the space below from settled sediment
  • Auxiliary scuba regulators for emergency cases (providing air from the inside?)
  • Exhale air channels on the ceiling: three channels leading divers’ exhaled air out of the access space to avoid accumulation. Therefore the ceiling is designed slightly tilted to the exit directions
  • Written procedure instructions on the walls. In advanced versions these plates can be replaced by screens to be able to customize shown information instantly.
  • Mounted benches to set aside materials
  • Outer gate to be moved from the inside or the access space automatically; and in case of emergencies manually.
  • Enough space for x divers inside the air filled part of the trunk to maintain enough space to communicate during technical failure
  • Air supply from the inside of the habitat controllable from the trunk
  • full communication devices between the air filled trunk and the inside of the habitat, including video surveillance and observation viewports
  • Not in the drawing: two facing but separated entrance complexes for unforeseen situations; trunks could be very close to each other to use the same wet room
  • Not in the drawing: comfortable stairway to reach the trunk and the habitat interior
Sketch for the access tunnel of the hangar concept

Hatches (external)

External Hatches function as interconnections between habitat and marine environment. They have to be able to resist preset pressure differentials when the are closed for lowering/lifting procedures, decompression or to prevent pressure changes due to tidal changes on the ocean surface. There are two kinds of different external hatches: main hatches (used regularly for crew and equipment) and emergency hatches (primarily for escape or rescue)

The following should be considered when locating and designing hatches:

  • None of the pressure hatches should be too massive or difficult to operate.
  • Because the pressure hatch’s function is critical, operating procedures and hardware should minimize the chance of unsafe operations. Normally, pressure hatch size and controls should be designed to be used by a suited crew member.
  • A dedicated emergency hatch should not interfere with normal activities. In an emergency, however, hatch operation should be simple and quick. Emergency hatches should be sized for suited crew members.

Types of Hatches

  • opening to the inside of habitat
  • opening to the outside of the habitat (into the trunk)
  • double hatches; one to each side

Size of Hatches

  • Each hatch must be sized to provide access for the largest crewmember. The size of equipment to be carried through a hatch should be considered. Hatches should require passage of crew members in diving suits.
  • Suited Crewmembers – All hatches must be large enough for ingress and egress of a suited crewmember.


  • Suited Operation – All opening and closing mechanisms must be operable by a single suited crewmember.
  • Unlatching – Hatches must require two distinct and sequential operations to unlatch, to prevent inadvertent opening of the hatch.
  • Securing – Hatches should be mechanically secured against sudden springing up when pressure differential has been failed to notice. Securing could be maintained by a simple metal chain mechanism.
  • Operation from Both Sides – Hatches must be capable of being opened, closed, latched, and unlatched from either side.
  • Without Tools – Hatches must be operable without the use of tools. Lost or damaged tools will prevent the hatch from being opened or closed, which may cause serious consequences.
  • Pressure Equalization – The ability to manually equalize habitat pressure from either side of each hatch must be provided.
  • Pressure Measurement – The ability to measure pressure difference across the hatch must be provided from either side of each hatch. This allows both land base personnel and mission crew to see if the pressure difference across a hatch is low enough to safely open the hatch. A pressure differential of the hatch was one of the reasons of a fatal accident in Sealab III leading to the termination of the multi-million dollar project.
  • Windows – Pressure hatches must have a window for direct visual observation of the environment on the opposite side of the hatch, to determine conditions or obstructions for safety purposes.
  • Status – The hatch closure and latch position status must be provided from either side of each hatch
  • Open Position – Hatch covers should be able to remain in the open position.
  • Restraints – Restraints must be provided as necessary to counteract body movement when opening or closing a hatch.
  • Operating Force – The forces required to operate hatch covers must be within the strength range of the weakest of the defined crewmember population, for the worst-case pressure differential anticipated.


  • Opened doors or hatch covers must not restrict the flow of traffic.
  • To avoid interference with translation, hatches should not be placed near a translation path juncture (including corners) and should be at least 1.5 m from a corner.
  • Hatch covers should not open into congested translation paths. Rather, they should open into a compartment.

Example Aquarius Reef Base

NEEMO 6 in the moonpool of Aquarius

In Aquarius there is no direct hatch over the trunk. The air pocket of the trunk consists of a complete room, the so called “wet room”, that is attached to the actual habitat. For decompression the hatch between the actual habitat and the wet room is locked, while the wet room always remains at ambient pressure. The following room can also be used as an airlock, e.g. if medical assistance is required during decompression. In this case the paramedic closes the hatch behind him after entering the air lock and opens the next hatch after the pressure between the airlock and the actual habitat is equalized. In normal conditions, however, the airlock is used as a standard working place. This is a very coherent system and very desirable solution for future ventures.

The Moonpool

The moonpool is the gateway between the habitat interior and the surrounding water. Its level is maintained by the internal pressure preventing water from rising and flooding the entire habitat. Pressure lower than ambient pressure would cause the level to rise, higher pressure would cause the air to bubble out from the trunk. Only by using a hatch between the moonpool and the habitat interior, different pressures can be applied, e.g. for lowering the habitat to the seafloor or for decompression in the end of a mission.

A moonpool on a ship

Moonpools are used in diving support vessels to lower/lift objects from the center of the vessel. (image from Wikimedia Commons: model of Ceona Amazon by Dr. Karl-Heinz Hochhaus)

There were several different types of moonpools. Smaller habitats used a huge pipe as a trunk ending in a horizontal hatch. Larger habitats like La Chalupa and Aquarius used (and still use) rectangle moonpools, while the “wet room” is separated from the actual habitat by a vertical door-hatch.

The moonpool of Hydrolab (see image at the beginning of this article) directly led to a very small airlock. Divers had to enter this airlock, close the hatch behind them and wait in a squatted position till the pressure equalization is completed to open the upper hatch and to enter the habitat.

The garage of Conshelf II
Conshelf II: Garage (2017)

The biggest moonpool to our knowledge was inside the “garage” of Conshelf II. The garage itself was separated by all means from the actual habitat. It served as a hangar for a submarine for two persons. It allowed the submarine to emerge and be lifted by a winch over the water, while the moonpool itself could be covered to allow maintenance of the sub. To reach the habitat the aquanauts had to dive, since the habitat and the garage were two different buildings. The moonpool was several meters in diameter. (Image from Wikimedia: Tauchsafari Sudan mit UMEX Tauchsport 2017 by wikiumex)

One specific difficulty with moonpools is the direct reaction of the water level to tidal changes on the sea surface. Water depth and pressure is changing if a wave passes above. This pressure has a direct effect on the volume of air inside the habitat leading to a change water level in the trunk/moonpool. Even a few centimeters would require the aquanauts to equalize their ears constantly. Closing the hatch would surely solve the problem. To open the hatch again there has to be a direct air connection between the air pocket in the trunk and the habitat interior to equalize the pressure in both compartments.

A curious thing happened in Sealab II: a great number of poisonous scorpion fish gathered in the trunk and hampered the divers from passing. What sounds like a funny incident could become a serious threat. Most likely they were attracted by the light coming from the wet room. To avoid similar situations in the future we should consider to install a mechanism that automatically turns the light inside the wet room into a red light (with the option to open normal light on demand) whenever the hatch is opened. This would accustom the divers eyes to the outside twilight and prevents incoming divers from glare. Additionally a distraction light on a less critical location could be activated prior to hatch activities to create an attraction to any marine animal.

Even if there are no information on pollution of the moonpool surface it is clear, that after a while there will be several particles swimming on the water, like hair, dust etc. Our proposal is to install an overflow mechanism that regularly pumps off the upper layer of the pool and disposes it to the outside the habitat.

The Wet Room

The wet room is located between the hatch and the main living and working area. Due to its natural high humidity it is well separated from the other areas by sealable doors. Ventilation is an important feature for the wet room. To avoid humidity built-ups there should be a procedure to regulate opening of the hatch if possible, especially in tropical waters, when the water temperature rises over the habitat interior temperature at night.

Showers are generally located in the wet room since the sewage is easily disposed over the entrance . One of the most desired features of former habitats was the availability of hot water to permit the aquanauts to warm up after excursions. Comfortable body temperatures are the top priority for successful missions. We should consider a separate room to maintain a certain degree of privacy. The WC will also be located in a separate cabin.

A rinsing basin for scuba gear should be available inside the wet room. After excursions this gear has to be cleaned from salt water to be stored for drying.

The scuba gear drying and storage facilities should be very well ventilated to avoid mold. In order to use the thermal discharge of compressors and machineries it may be a good idea to position the storage close to the separating wall of the machinery section. Maybe there is a possibility to lead used (warm) air from the habitat interior through the storage area before it arrives at the scrubber devices. It is important that the storage area is large enough to avoid lack of ventilation around the single items.And there should be enough space to change clothes without hindering any other procedure.

Besides there should be locker space for individual diving equipments and to store dry clothes while the aquanaut is outside the habitat.

A winch over the moon pool is necessary to lift heavy items out of the water. This could be daily cargo, heavy machinery spare parts or simply diving gear.

General Consideration

Like in all major habitats there should be alternative exits in different parts of the habitat for the case, that the main entrance is blocked for any reason. These can be hidden hatches, but they have to be well marked, always accessible and regularly controlled. Of course, there should be corresponding diving equipments nearby for emergency exits.

Aquanauts of Sealab requested outside cameras to oversee activities around the trunk. They are vital for the coordination of emergencies.

The most desirable moonpool would look like the one  seen in the movie Abyss. A large pool for all kinds of submarines, equipments and divers. Well, beside the difficulty of balancing the structure, compensating the tidal changes,… and the money… why not?


Here is a summarizing table of all required features of the entrance complex. I divided it into two different habitat types: the experimental habitat, which is small and mobile; and Module 1 of the large hangar shaped structure. Primary features are necessary from the beginning; secondary features are subject to addition afterwards.


(small mobile version)
primary features
- distraction light on habitat exterior
- hooks and shelves on exterior
- exterior camera for supervision from inside
- guidance light next to trunk entrance
- sufficient underwater access space (>=1m)
- sufficient trunk diameter (≈120cm)
- sufficient air pocket (>=70cm)
- handles, foot leans, sitting installations
- light inside the air pocket
- communication installation
- single hatch (opens to the outside)
- additional emergency hatch
- hatch decompression valve (?)
- safety measures for hatch handling
- ladder to the wetroom

- shower (=rinsing facility)
- standard boat WC (separate cabin)
- sink, mirror, electric socket
- hooks, shelf

- personal locker
- equipment drying/storage installation
- dehumidification facilities
- automated red light upon hatch opening
- optional white light switches
- (mountings for secondary features)

secondary features
- emergency breathing installations
- moonpool overflow for surface cleaning
- hot shower (=rinsing facility)
- winch
Module 1
(of large structure, hangar type)
primary feaatures
- distraction light on habitat exterior
- hooks and shelves on exterior
- exterior camera(s) for supervision from inside
- guidance light next to trunk entrance
- sufficient underwater access space (>=1,50m)
- sufficient trunk diameter (≈200cm)
- large air pocket (≈200x300cm, ≈150cm height)
- sitting facilities, grating (≈200x80cm)
- scuba tank securing installation
- lighting inside the air pocket
- communication installation
- two mutual hatches
- additional emergency hatch(es)
- hatch decompression valves (?)
- safety measures for hatch handling
- emergency breathing installations
- stairway to the wetroom

- hot shower (=rinsing facility)
- shower cabin
- WC in separate cabin
- sink, mirror, electric socket
- hooks for towels and clothes
- shelves for personal belongings
- storage locker

- personal locker
- equipment drying/storage installation
- sealable door to other sections
- dehumidification facilities
- automated red light upon hatch opening
- optional white light switches
- (blue light when wetroom is not in use)
- (mountings for secondary features)

secondary features
- "shark cage" (if necessary)
- scuba tank refilling installation
- airlock
- moonpool overflow for surface cleaning
- rinsing basin
- advanced drying installation

This article needs to be improved. Help us by using the comment box below.

Still to be added:

  • sketch of the different entrance sections
  • Link the regular maintenance procedures of the emergency exits to the maintenance manual.

This article contains adaptations from the NASA Human Integration Handbook (HIDH), NASA/SP-2010-3407. NASA copyright policy states that “NASA material is not protected by copyright unless noted”. (See NASA copyright policy page or JPL Image Use Policy.)

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