God Slayers - Page 176 (Patreon)

Undergrounders, or 地底人 (Chisokojin) as they might call themselves, thrive in the labyrinthine tunnels beneath the surface of Kuserra not because they are excellent diggers, but because they are capable of squeezing through narrow or collapsed passages with great ease. They are impressively flexible, and can squeeze their whole body through any hole that's wide enough to accommodate their skulls. Additionally, the soft light given off from their eyes combined with a hypersensitivity to low light allows them to see clearly in total darkness. As a result, they live quite comfortably in narrow and claustrophobic spaces beneath the crumbling towers up above.

Naturally, in a place like Kuserra, where holes in buildings are the rule, not the exception, this gives Undergrounders an advantage when trying to get into (or out of) places they shouldn't be. This causes many on the surface to look down on them as vermin.