Researchers just finished a successful field test with a robotic submarine that can autonomously study the ocean for up to 6 months at a time. While it uses battery reserves for its more fundamental navigation and communication systems, the torpedo-like glider by Webb Research Corporation and WHOi can propel itself using the temperature differences within the ocean.
It's a neat idea that seems straight out of Mr. Wizard. When gliding through warm water, internal chambers filled with wax expand, repositioning internal oil bladders and changing the glider's buoyancy—causing it to sink. Then cold water at lower depths causes the wax chamber to contract, moving the oil back in place allowing the system rises again.
As we understand it, this principle alone would simply make the device go up and down within the water. But adding wings to the torpedo allows it to harness this up down pressure and glide through the water.