From the Astronomy Picture of the Day, it’s the remnants of a two-billion-year-old nuclear reactor discovered in 1972 in a mine in Oklo, Gabon.
Apparently in the old days there was enough uranium-235 in the Earth’s crust that, under the right conditions, nuclear fission could occur naturally. Over time the fuel was used up, and now uranium deposits are mostly 238U, so we don’t need to worry about any new nuclear reactors popping up without our help.
What’s really odd is that this reactor produced plutonium naturally. There’s still some there. Most periodic tables I’ve seen label plutonium as a synthetic element, so the idea of natural plutonium takes some getting used to.
Kind of like the idea of a natural nuclear reactor.