Nuclear reactors could be burning deep beneath the ground, two scientists have claimed. They say that uranium could become sufficiently concentrated at the base of Earth’s mantle to ignite self-sustained nuclear fission, as in a human-made reactor.

This is not the first time that natural ‘georeactors’ deep inside Earth have been proposed, and the idea has previously been greeted with scepticism by geoscientists. But physicist Rob de Meijer of the University of the Western Cape in Cape Town, South Africa, and geochemist Wim van Westrenen of the Free University of Amsterdam in the Netherlands, believe that their new proposal¹ is more plausible.

Spontaneous ignition

Yet it is clear that natural nuclear reactors can occur. Crustal rocks at Oklo in Gabon, Africa, bear unambiguous evidence of spontaneous ignition of uranium fission in mineral deposits 1.7 billion years ago.