This article considers Kentucky’s Mammoth Cave as an unexpected site for nineteenth-century theorizations of racialized Blackness. Mammoth Cave became a major tourist attraction in the 1840s, generating a host of guidebooks, travel accounts, magazine illustrations, panoramas, newspaper articles, and fiction. Crucial to its fame was the fact that the guides who led visitors through the cave were enslaved men. This article argues that white writers responded to the guides’ knowledge of the cave by reframing it as affinity. In doing so, they transformed Mammoth Cave’s subterranean darkness into a manifestation of racialized Blackness. But the writers’ racialization of Mammoth Cave also had a tendency to slip out of their control. As they associated its spatial darkness with racialized Blackness, the literal underground of Mammoth Cave flickered into an underground that was more than literal—a mysterious Black formation, of unguessed dimensions and certain danger, beneath the world as they knew it. Finally, the article asks what we can glean from the literature of Mammoth Cave about the body of Black thought it sought to disavow: the alternative relations between race and the underground that the guides theorized through their own subterranean explorations.