A Bathysphere is a character with good armor class, hit points, and saving throws; a Dwarf with a few levels under their belt and magic armor is an excellent candidate, especially with their Infravision. If you have a 3rd level Magic-User, throw an Invisibility spell on the Bathysphere.
The function of the Bathysphere is to serve as an "Iron Scout"; whereas conventional dungeoneering scouting wisdom involves the use of a stealthy character, the problem with using a Thief for scouting is that they usually have crap hit points, leather armor, unreliable stealth skills, and usually require some form of light source - the result being that when a thief style scout inevitably encounters monsters or a trap they have a fair chance of ending up dead.
In contrast the Bathysphere is durable, a tough bugger that has a good chance of surviving traps and encounters; and if the monsters or trap are especially lethal, the Bathysphere dies instead of the entire party. Whereas the traditional Thief scout can be a tissue paper pinata, the Bathysphere is a freaking Bomb Disposal Robot.
While dungeoneering, the group that I play in has utilized Dwarven Bathyspheres to great effect...instead of a cowering, fragile Thief scout they're highly effective as bold, confident, durable scouts; whereas sending a Thief on a solo scouting mission can be a callous gamble, the Bathysphere has a damn good chance of making it back with the intel and/or loot in one piece.