Diving in a Heavy-Walled Vessel
Heavy-walled vessels can withstand tremendous underwater pressures and maintain their internal pressure at or near sea level pressure -- a pressure called "one atmosphere" or "one atm."

Such vessels include: the bathysphere -- an unpowered hollow steel ball lowered from the surface support ship by steel cable; the bathyscaphe -- a bathysphere with buoyancy control so that cable is not needed for descent and ascent; and the submarine -- which can travel great distances in any direction under its own power.

All these vessels require a system to provide fresh air (usually by adding oxygen to the existing air) and to get rid of exhaled carbon dioxide (with soda lime, lithium hydroxide, or similar chemical compounds that take up carbon dioxide). A modern extension of the one-atmosphere vessel is the self-contained armored diving suit, flexible yet able to withstand pressures at depth. In effect, a diver in one of these armored suits becomes almost like a small submarine.