But for those who live near the ocean, it’s not the wind or rains that scare them, it’s the storm surge.

That’s a term that might be unfamiliar to people who live inland. Simply put: A storm surge is an abnormal rise in water levels that can be caused by a major storm. Winds from the storm, including external feeder bands, push water toward shore. But instead of simply cascading in and out, as a normal wave would, these waters build on top of each other.

Those can accumulate and cause flooding—especially during a normal high tide. Adding to the threat, the water level rises associated with a storm surge do not include waves whipped up by the storm’s winds. And with much of the Atlantic and Gulf Coast coastlines less than 10 feet above mean sea level, that’s especially worrisome.

Hurricane Irma, because of its size, is causing some massive storm surges along both the Atlantic and Gulf coasts. Early Monday, Jacksonville, Fla. broke its all-time record, with storm surges bigger than Hurricane Dora in 1964 and is under a Flash Flood Warning until 12:45 pm ET. The city saw an extra five feet of water.

Charleston, S.C. is expecting high tide just after noon today and forecasters warn that the tide height will crest at over 10 feet, causing extensive flood problems through town.

Waves breaking over the wall at the Battery in downtown Charleston. 🌊💨 #Irma🌀 pic.twitter.com/XsN9r5rJaJ

— Trooper Bob (@TrooperBob_SC) September 11, 2017

For reference, that would top the storm surge of last year’s Hurricane Matthew, but fall short of 1989’s Hurricane Hugo, which devastated the town.

The National Hurricane Center notes storm surge “is often the greatest threat to life and property from a hurricane.” It was, in fact, storm surges that made Hurricane Katrina so devastating in New Orleans. That storm saw surges that were 25 to 28 feet above normal tide levels.