Then, a wall of water, with an unimaginable force that can flatten an entire building in seconds.
If you survive the initial blow, you now face an unrelenting pounding of wave after wave, and like a pinball machine, you are being battered by debris, uprooted trees and cars that share the same violent trajectory as you.
Those who have lived through a tsunami describe an almost unsurvivable scenario.
"The first wave just looked like a wall of white coming toward you, and the sound ... it was just screams everywhere and just the crashing sound. .. the water just kept coming and coming," one tsunami survivor told CNN.
So how do people survive one of Mother Nature's most deadly forces?
Get to higher ground
Tsunamis are giant waves triggered by anything that disrupts the ocean, including earthquakes or volcanic eruptions under the sea, so the first sign may be a rumbling Earth, or a rapid rise or fall in ocean waters.
Don't wait for a warning. Run.
For people in low-lying, coastal areas, a tsunami could arrive just minutes after an earthquake, far too quickly for official warnings to be disseminated.
"Similar to storm surge in a hurricane, if you are on the beach when a tsunami comes ashore, you would not survive it," said CNN meteorologist Brandon Miller.
"The force of the water, as well as all the turbulence and currents within, would be too strong. A 6-10 foot tsunami, while it doesn't sound huge, would be extremely damaging, and can run ashore great distances, depending on the elevation of the coastline," Miller said.
Too late to run. Now what?
Sometimes it may be impossible to avoid a tsunami.
If you are trapped or unable to evacuate to higher ground, your next step is to find a sturdy vertical refuge.
This is not ideal as the structure itself could collapse, but if it is your only course of action, head to the highest possible point.
In areas with a heightened threat of tsunami, you may see buildings or escape routes marked with the international symbol for tsunami, follow these.
If you are swept up in a turbulent current, your best course of action is to cling to something that floats, such as a door or a tree trunk.
"I was saved by holding onto the roof, but my wife was swept away," said one man
who survived the deadly 2011 tsunami in Japan.
Do not return to coastal areas until the tsunami threat is declared over by authorities.
Pay close attention to your local alert system -- radio announcements, official websites, social media accounts and emergency cellphone alerts -- and heed the advice.
Tsunamis can last for hours, so use common sense.
During the 2004 Indian Ocean tsunami, the most devastating on record, many people were killed when they returned to the beach to watch the retreating ocean exposing the seafloor.
A rapidly receding ocean is a phenomenon that usually precedes a killer wave, according to NOAA