The goal of this paper is to develop an underwater robot with a buoyancy control system based on the spermaceti oil hypothesis. Sperm whales have a spermaceti organ in their head that is filled with spermaceti oil. Spermaceti oil is high quality oil and was used as material for candles, lubricant, and so on. There is a hypothesis about spermaceti oil that insists that sperm whales melt and congeal their spermaceti oil and change the volume of the oil to control their own buoyancy. This hypothesis appears suitable for the underwater robot because no materials for the ballast, such as sea water taken in at another place and iron, are discarded in the sea. To choose the best material as a spermaceti oil substitute, we measured the densities of four materials at both liquid and solid states, and calculated their buoyancy differences between both states. From the results, we concluded that the paraffin wax was the best material because its buoyancy difference is the largest of the four and its melting point is relatively low. Next, we directly measured the buoyancy of the paraffin wax and found that a particular arrangement of nichrome wire, which heats the oil, increases the level of buoyancy. Finally, we developed an underwater robot with a buoyancy control system based on this hypothesis. We measured its buoyancy and succeeded in surfacing of the robot in a small water tank.

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