Reggel in Zoids Genesis
Mii explains, if you dig deep enough, you'll find water, Zoids and Reggel.
Reggel is the energy resource used to power Zoids in the Zoids: Genesis anime series. It has the consistency of oil and is produced in giant, tree-like structures they call "Generators". Orange-Red in color, Reggel looks like magma or lava. Furthering the connection to lava, Reggel does occur naturally (outside of generators) in episode 10 of Zoids: Genesis, Ron explains how it can be found near volcanoes or hot springs. Both of which are sources of lava. However, naturally forming Reggel is lower in quality than Reggel formed in a generator, yet it is still good enough for Zoids to use. Additional information is given in episode 28; Rei Mii explains that Reggel can be found by digging into the ground. An on-screen diagram depicts Reggel coming from Volcanoes.
Reggel is poured into Zoids via a small hatch, although it is implied wild Zoids can consume Reggel. Not much is needed for a Zoid to run. Usually about enough to fill a bucket is used to activate a large-scale Zoid.