From the main mine that humans made in the Oklo region, one of the natural reactors is accessible via an offshoot, as illustrated here. (UNITED STATES DEPARTMENT OF ENERGY)

Earth’s First Nuclear Reactor Is 1.7 Billion Years Old And Was Made Naturally

Planets can ‘discover’ nuclear power on their own, naturally, without any intelligence. Earth did it 1.7 billion years before humans.

If you were hunting for alien intelligence, looking for a surefire signature from across the Universe of their activity, you’d have a few options. You could look for an intelligent radio broadcast, like the type humans began emitting in the 20th century. You could look for examples of planet-wide modifications, like human civilization displays when you view Earth at a high-enough resolution. You could look for artificial illumination at night, like our cities, towns, and fisheries display, visible from space.

Or, you might look for a technological achievement, like the creation of particles like antineutrinos in a nuclear reactor. After all, that’s how we first detected neutrinos (or antineutrinos) on Earth. But if we took that last option, we might fool ourselves. Earth created a nuclear reactor, naturally, long before humans ever existed.

Reactor nuclear experimental RA-6 (Republica Argentina 6), en marcha, showing the characteristic Cherenkov radiation from the faster-than-light-in-water particles emitted. The neutrinos (or more accurately, antineutrinos) first hypothesized by Pauli in 1930 were detected from a similar nuclear reactor in 1956. (CENTRO ATOMICO BARILOCHE, VIA PIECK DARÍO)