Earth batteries have been used off and on for various purposes since at least the 1840s with one version or another dating back a few centuries. So what is an earth battery? Well, it is basically taking two dissimilar metals, such as iron and copper, burying them in the ground, or in a container with moist soil, and allowing the natural current created to transfer to your metal electrodes. They are similar to potato batteries or lemon batteries.
The soil, when wet, acts as an electrolyte solution similar to those you could find in other liquid-based batteries. So once you have your electrodes in place, and some copper wire to connect them, you have yourself a battery system that can easily generate around 5 volts, with the potential for much more with some experimentation.
While you aren’t likely to be able to generate enough power to run things like clothes dryers, you can easily create enough power to run some lights, charge your cell phone or run other smaller appliances. While the batteries don’t last forever due to the soil eventually running out of electrolytes, you can just replace the soil and you are back in business.
These could be of use by simply keeping the basic components, which could be as little as galvanized nails and copper wire in your backpack or in your home to use to charge a cell phone or run a few lights in case of a power outage or other emergency. Who knows, maybe someone out there will design a system capable of powering much more. It seems like the type of challenge those who live off-grid would be up for.