A bathysphere was a large metal sphereical container with thick glass windows. It was designed for viewing creatures of the sea at depths no human could survive, as its metal frame protected the diver inside from the crushing pressure of the water. It was invented by Jacques Douteux and presented as one of the exhibits at the Hill Valley Science Expo of 1931.
At the science expo, the bathysphere was placed in a large tank of water and could be raised or lowered by turning a circular handle.
Sometime during the expo, an Emmett Brown from an alternate timeline took Jacques Douteux's place and hid his unconscious younger self in the bathysphere, feeding an air hose into it in order to keep him alive. Marty McFly was onto Brown and, after the latter's attempts to ward him off, stepped on the hose, cutting off Emmett's air supply. After a lengthy standoff in which Brown refused to raise the bathysphere despite his younger self losing air - consequently causing him to suffocate as well - and McFly refused to uncrimp the hose, Brown finally gave in and raised the bathysphere. Marty opened it, letting air in, and freed the young Emmett. Brown fled as this happened, knowing an expo official had watched the entire exchange and Brown would soon be in deep trouble.