So-called microbial fuel cells have been around for more than a century, and their operating principle is similar to batteries with an anode, cathode and electrolyte. Instead of generating electricity from chemical sources, they use bacteria, which share electrons as they absorb soil.

The problem was maintaining water and oxygen levels in them when they were buried underground, so a team of American scientists came up with several design options to provide the cells with access to oxygen. The version in the form of a cartridge on top of a horizontal disk was recognized as the best. A disk-shaped carbon anode sits at the bottom of the device, buried deep in the ground, while a conductive metal cathode sits on top of the anode.

The developers note that the lower part of the cathode is at a sufficient depth to gain access to moisture deep in the soil, and the upper part, in turn, is located at the same level with the surface. The device is designed in such a way as to protect it from dirt, and parts of the cathode are made of waterproof material to protect it from flooding. During tests, the device generated 68 times more energy than it needed to power its own systems.