A diving bell is a rigid chamber used to transport divers from the surface to depth and back in open water, usually for the purpose of performing underwater work. The most common types are the open-bottomed wet bell and the closed bell, which can maintain an internal pressure greater than the external ambient. Diving bells are usually suspended by a cable, and lifted and lowered by a winch from a surface support platform. Unlike a submersible, the diving bell is not designed to move under the control of its occupants, or to operate independently of its launch and recovery system.
The wet bell is a structure with an airtight canopy which is open to the water at the bottom, that is lowered underwater to operate as a base or a means of transport for a small number of divers. Air is trapped inside the bell canopy by pressure of the water at the interface. These were the first type of diving chamber, and are still in use in modified form.
The closed bell is a pressure vessel for human occupancy, which may be used for bounce diving or saturation diving, with access to the water through a hatch at the bottom. The hatch is sealed before ascent to retain internal pressure. At the surface, this type of bell can lock on to a hyperbaric chamber where the divers live under saturation or are decompressed. The bell is mated with the chamber system via the bottom hatchway or a side hatchway, and the trunking in between is pressurized to enable the divers to transfer through to the chamber under pressure. In saturation diving the bell is merely the ride to and from the job, and the chamber system is the living quarters. If the dive is relatively short (a bounce dive), decompression can be done in the bell in exactly the same way it would be done in the chamber.
A third type is the rescue bell, used for the rescue of personnel from sunk submarines which have maintained structural integrity. These bells may operate at atmospheric internal pressure and must withstand the ambient water pressure.
History
The diving bell is one of the earliest types of equipment for underwater work and exploration. Its use was first described by Aristotle in the 4th century BC: "They enable the divers to respire equally well by letting down a cauldron, for this does not fill with water, but retains the air, for it is forced straight down into the water." There are apocryphal accounts of Alexander the Great (including some versions of the Alexander Romance), exploring underwater in an enclosed vessel, lowered from his ships. Their origin is hard to determine, but some of the earliest dated works are from the early Middle Ages.
In 1535, Guglielmo de Lorena built and tested a diving bell, to explore a sunken vessel in a lake near Rome. De Lorena's diving bell only had space for enough oxygen for a few minutes. However, the air in his diving bell was reported to last for one to two hours with the limiting factor being a diver's ability to withstand cold and fatigue, not lack of oxygen. The mechanism he used needed to keep the pressure inside the bell continuous, supply fresh air, and remove air exhaled by the diver. To accomplish this, it is believed that de Lorena used a system similar to Edmond Halley's design, two and a half centuries later.
An improved, but possibly unbuilt, design from 1616 by Franz Kessler featured a bell covering a diver down to their ankles, and added windows and a ballast to the bottom. This design supposedly no longer needed to be tethered to the surface, although there is no evidence it was actually built.
In 1642, John Winthrop reported one Edward Bendall building two large wooden barrels, weighted with lead and open at their bottoms, to salvage a ship Mary Rose which had exploded and sunk, blocking the harbor of Charlestown, Boston. Bendall undertook the work on condition that he be awarded all the value of the salvage should he succeed in unblocking the harbor, or half the value he could salvage if he could not.
In 1658, Albrecht von Treileben was permitted to salvage the warship Vasa, which sank in Stockholm harbor on its maiden voyage in 1628. Between 1663 and 1665 von Treileben's divers were successful in raising most of the cannon, working from a diving bell.
A diving bell is mentioned in the 1663 Ballad of Gresham College (stanza 16):
In late 1686, Sir William Phipps convinced investors to fund an expedition to what is now Haiti and the Dominican Republic to find sunken treasure, despite the location of the shipwreck being based entirely on rumor and speculation. In January 1687, Phipps found the wreck of the Spanish galleon Nuestra Señora de la Concepción off the coast of Santo Domingo. Some sources say they used an inverted container for the salvage operation while others say the crew was assisted by Indian divers in the shallow waters. The operation lasted from February to April 1687 during which time they salvaged jewels, some gold and 30 tons of silver which, at the time, was worth over £200,000.
In 1689, Denis Papin suggested that the pressure and fresh air inside a diving bell could be maintained by a force pump or bellows. Engineer John Smeaton utilized this concept in 1789.
In 1691, Dr. Edmond Halley completed plans for a diving bell capable of remaining submerged for extended periods of time, and fitted with a window for the purpose of undersea exploration. In Halley's design, atmosphere is replenished by sending weighted barrels of air down from the surface.
In 1775, Charles Spalding, an Edinburgh confectioner, improved on Halley's design by adding a system of balance-weights to ease the raising and lowering of the bell, along with a series of ropes for signaling the surface crew. Spalding and his nephew, Ebenezer Watson, later suffocated off the coast of Dublin in 1783 doing salvage work in a diving bell of Spalding's design.
Mechanics
The bell is lowered into the water by cables from a crane, gantry or A-frame attached to a floating platform or shore structure. The bell is ballasted so as to remain upright in the water and to be negatively buoyant, so that it will sink even when full of air.Britannica Editors. "diving bell". Encyclopedia Britannica, 9 Dec. 2011, https://www.britannica.com/technology/diving-bell-submersible-vessel. Accessed 6 December 2025.
Hoses, supplied by gas compressors or banks of high pressure storage cylinders at the surface, provide breathing gas to the bell, serving two functions: Fresh gas is available for breathing by the occupants. Volume reduction of the air in an open bell due to increasing hydrostatic pressure as the bell is lowered is compensated. Adding pressurized gas ensures that the gas space within the bell remains at constant volume as the bell descends in the water. Otherwise the bell would partially fill with water as the gas was compressed.
The physics of the diving bell applies also to an underwater habitat equipped with a moon pool, which is like a diving bell enlarged to the size of a room or two, and with the water–air interface at the bottom confined to a section rather than forming the entire bottom of the structure.
Wet bell
A wet bell, or open bell, is a platform for lowering and lifting divers to and from the underwater workplace, which has an air filled space, open at the bottom, where the divers can stand or sit with their heads out of the water. The air space is at ambient pressure at all times, so there are no great pressure differences, and the greatest structural loads are usually self weight and the buoyancy of the air space. A fairly heavy ballast is often required to counteract the buoyancy of the airspace, and this is usually set low at the bottom of the bell, which helps with stability. The base of the bell is usually a grating or deck which the divers can stand on, and folding seats may be fitted for the divers' comfort during ascent, as in-water decompression may be long. Other equipment that is carried on the bell includes cylinders with the emergency gas supply, and racks or boxes for tools and equipment to be used on the job. There may be a tackle for hoisting and supporting a disabled diver so that their head projects into the air space.
Type 1 wet bell The type 1 wet bell does not have an umbilical supplying the bell, because diver's umbilicals supply the divers directly from the surface, similar to a diving stage. Divers deploying from a type 1 bell will exit on the opposite side to where the umbilicals enter the bell so that the umbilicals pass through the bell and the divers can find their way back to the bell at all times by following the umbilical. Bailout from a type 1 bell is done by exiting the bell on the side that the umbilicals enter the bell so they no longer pass through the bell, leaving the divers free to surface.
Type 2 wet bell A gas panel inside the bell is supplied by the bell umbilical and the emergency gas cylinders, and supplies the divers' umbilicals and sometimes built-in breathing system (BIBS) sets. There will be racks to hang the divers' excursion umbilicals, which for this application must not be buoyant. Abandonment of a type 2 wet bell requires the divers to manage their own umbilicals as they ascend along a remaining connection to the surface.
Operation of a wet bell
The bell with divers on board is deployed from the working platform (usually a vessel) by a crane, davit, or other mechanism with a man-rated winch. The bell is lowered into the water and to the working depth at a rate recommended by the decompression schedule, and which allows the divers to equalize comfortably. Wet bells with an air space will have the air space topped up as the bell descends and the air is compressed by increasing hydrostatic pressure. The air will also be refreshed as required to keep the carbon dioxide level acceptable to the occupants. The oxygen content is also replenished, but this is not the limiting factor, as the oxygen partial pressure will be higher than in surface air due to the depth.
When the bell is raised, the pressure will drop and excess air due to expansion will automatically spill under the edges. If the divers are breathing from the bell airspace at the time, it may need to be vented with additional air to maintain a low carbon dioxide level. The decrease in pressure is proportional to the depth as the airspace is at ambient pressure, and the ascent must be conducted according to the planned decompression schedule appropriate to the depth and duration of the diving operation.
Closed bell
A closed, or dry, bell, also known as a personnel transfer capsule or submersible decompression chamber, is a pressure vessel for human occupancy which is lowered into the sea to the workplace, equalised in pressure to the environment, and opened to allow the divers in and out. These functional requirements dictate the structure and arrangement. The internal pressure requires a strong structure, and a sphere or spherically ended cylinder is most efficient for this purpose. When the bell is underwater, it must be possible for the occupants to get in or out without flooding the interior. This requires a pressure hatch at the bottom. The requirement that the bell reliably retain its internal pressure when the external pressure is lowered dictates that the hatch open inward, so that internal pressure will hold it closed. The bell is lowered through the water to working depth, so must be negatively buoyant. This may require additional ballast, which may be attached by a system that can be released from inside the bell in an emergency, without losing pressure, to allow the bell to float back to the surface.