A 9.0-magnitude earthquake rips through the Pacific Northwest. Roads split. Buildings crumble. Fifteen minutes later, a massive tsunami crashes onto shore, increasing the damage by a factor of ten. This is a worst-case scenario—but according to seismologists, the question of some kind of earthquake tearing up the Cascadia Subduction Zone, which lays under Oregon, Washington State, and British Columbia, is less an “if” than a “when.”

What do you do when faced with that kind of future? If you’re in the disaster response business, you practice.

Starting today, 20,000 people—including soldiers, emergency managers, and ham radio operators—will roleplay a massive earthquake and tsunami scenario. The four-day exercise, called “Cascadia Rising,” will be the largest emergency management drill in the history of the Pacific Northwest.

The drill is designed to test how well different players can work together, especially when facing major transport blockages and communication knockouts, ABC News reports. Soldiers will deliver supplies to Washington’s Vashon Island, and faux-rehabilitate the Port of Tacoma. Members of Amateur Radio Emergency Services, a ham radio NGO, will practice leapfrogging phone and internet outages to connect rescue teams with each other. Other groups don’t know quite what they’re in for, and will have to stay on their toes.

Odds are there won’t be much time for tweeting, but interested parties can check out the drill’s progress with the hashtag #CascadiaRising.

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