The threat? A massive earthquake along a sleeping giant known as the Cascadia subduction zone — and rising sea levels might make things even worse.

What is the Cascadia subduction zone?

Stretching from Northern Vancouver Island to Cape Mendocino, California, this fault line has the potential to unleash an enormous quake, triggering not only land collapse but a catastrophic wave event capable of swallowing entire towns.

A new study published in the Proceedings of the National Academy of Sciences breaks down the danger: researchers from Virginia Tech say a quake in this area could cause land to sink by up to 6½ feet, dramatically expanding coastal floodplains.

“The expansion of the coastal floodplain following a Cascadia subduction zone earthquake has not been previously quantified,” explained Tina Dura, lead author of the study and assistant professor at Virginia Tech.

“The impacts to land use could significantly