Abstract: In accordance with the invention, a hollow, gas-containing diving body is disposed in a partially shaded transparent column partially-filled with a clear liquid. The diver is weighted for near-neutral buoyancy. The upper portion of the column is shaded, and the lower portion is subjected to thermal radiation such as sunlight. When radiation strikes the diver, it heats the gas, thereby increasing the gas volume, displacing liquid and increasing the buoyancy of the diver. The diver therefore rises to the shaded region where the gas cools, causing the diver to sink. The cycle repeats.