While trying to make a microbial fuel cell apparently I stumbled upon how to make an Earth battery, which is something whose existence I really had no clue about until I made one. While the current it generates is pretty much non-existent I've found that several of these in series are capable of dimly lighting a 3 volt LED for weeks, which I still think is pretty good for what amounts to a jar of mud.
To make this "mud battery" you'll need:
An Empty Container
Insulated Copper Wire
At Least 5 cm of Graphite (taken from a pencil)
A Digital Multimeter
A Hand Shovel
Step 1: Obtain and Insert the Dirt
For this step, just go outside and find a spot where you can dig up some dirt. Though I've yet to experiment with how various types of dirt affect the mud battery's function, I've found while making the some dozen cells I've tried that the blackish-brown sort of topsoil pictured works well. If I had to guess, I'd say the dirt I use is towards the "loamy" side of things because it retains water well without holding it up so much that it prevents the diffusion of ions between the electrodes
Besides finding the right kind of soil to use, this step really is easy as just filling up your container with dirt.
Step 2: Prepare the Anode
To prepare the anode strip a piece of wire such that one end has a little bit of copper exposed (~2 cm) while the other has significantly more copper exposed (~4 cm). Now take a piece of aluminum foil and wrap it around the long end of the wire so as to form a sort of "pin" of aluminum foil. Once finished stick the anode into the dirt.