question:  Why did Loy Chuk's people live underground?
context: A scream of brakes, the splash into icy waters, a long descent into alkaline depths ... Movement was slower now, as he began to sink, trapped inside his wrecked car. The Pit was a tremendously deep pocket in the ground, spring-fed. The edges of that almost bottomless pool were caked with a rim of white—for the water, on which dead birds so often floated, was surcharged with alkali. Presently, when he could no longer hold his breath, bitter liquid was sucked into his lungs. The ripples that had ruffled the surface waters in the Pit, quieted again to glassy smoothness. Lacking qualities identifiable as human, the disturbance was still like the babble of a group of workmen who have discovered something remarkable. The icy breeze tore tiny puffs of dust from grotesque, angling drifts of soil, nearly waterless for eons. A launching catapult for the last space ships built by the gods in exodus, perhaps it was—half a million years ago. Glacial ages, war, decadence, disease, and a final scattering of those ultimate superhumans to newer worlds in other solar systems, had done that. Loy Chuk had flown his geological expedition out from the far lowlands to the east, out from the city of Kar-Rah. Loy Chuk and his fellow workers were gathered, tense and gleeful, around the things their digging had exposed to the daylight. The dry mud that had encased it like an airtight coffin, had by now been chipped away by the tiny investigators; but soiled clothing still clung to it, after perhaps a million years. A mineral saturation that had held time and change in stasis. Not the star-conquering demi-gods, but the ancestral stock that had built the first machines on Earth, and in the early Twenty-first Century, the first interplanetary rockets. A strange accident, happening in a legendary antiquity, had aided them in their quest for knowledge. Medical and biological sciences were far advanced among Loy Chuk's kind.
output:  The temperature above ground at night is very cold.
options:  Subterranean passages protect against desert sand storms.  Subterranean passages protect against larger predators.  Loy Chuk comes from a rodent species. Rodents usually live in underground burrows.  The temperature above ground at night is very cold.