A diving bell was an early system for underwater diving. 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 breathe, 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 1600s. Scuba diving has mostly replaced diving bells although they are still used sometimes.