Topography seems easy to explain when beholding jagged summits such as Colorado’s Rocky Mountains or California’s Sierra Nevada range. After all, these mountains mark spots where the continent grew thick during violent collisions with other tectonic plates: the land crumpled and heaved skyward, like the hood of a car buckling in a head-on crash.

But surface-shaping geologic forces account for surprisingly little of the planet’s highs and lows. About half of North America’s elevation actually results from the planet’s internal heat. Minus that warmth, most land would sink below sea level, say University of Utah geophysicists Derrick Hasterok and David S. Chapman. Based on their recent calculations, parts of the Rockies and high Sierras would shrink to mere islands, whereas cities such as New York, Los Angeles and even mile-high Denver would slip 200 meters (700 feet) or more underwater.