A diving bell was an early system for diving underwater. The basic idea of a bell is that a hollow object, even with an open bottom, will hold air when put under water (this can be seen with a glass in a sink). If the bell is heavy enough it will sink below the water. A swimmer can be inside the bell and breath, or hold his breath and swim outside the bell for a short time, and come back to the bell to breath again. The idea had been around for a long time, but became useful in the 1600's. Scuba diving replaced it.