For relief workers in Nepal after the massive earthquake on April 25, one of the challenges is just knowing where to go: Most roads and buildings don’t exist on a map. But that’s a situation that’s changing, hour by hour, as thousands of volunteers around the world build a detailed digital atlas of the earthquake zone as part of the Humanitarian OpenStreetMap Team (HOT).

Volunteers use aerial images from satellites to mark open spaces where helicopters or planes might land with supplies, highlight streets between towns and villages, and outline buildings that aid groups can use to guess where victims might be. Using OpenStreetMap technology–known as the “Wikipedia of maps”–they build continuously updated maps that can be used online or downloaded into navigation devices.

72 hours after, thousands of amateurs have mapped something like 30,000 buildings.

Just two hours after the earthquake hit, the organization’s coordinators in the U.S. were getting emails from a partner group in Nepal, Kathmandu Living Lab. “They laid out districts and villages that needed mapping because they were getting all of these reports of awful damage and casualties,” says Blake Girardot, activation coordinator of HOT and vice president of the organization’s board.

Thanks to some foresight, the group had already worked on maps of most of the capital city. “Half of our mission is response to crises like this, so when something awful happens we can jump on it and start giving them data they need,” says Girardot. “But the other half of the equation is preparing for these disasters. It’s not a mystery where vulnerable places are. In places with earthquakes, floods, drought, or political conflict, we can start identifying those places ahead of time and start doing mapping before the crisis happens.”

Since Nepal was identified as a vulnerable location by the World Bank and other organizations, HOT began mapping Nepal a few years ago, and helped set up Kathmandu Living Lab to work on the project locally. The group is working quickly to map out the remaining areas of the disaster zone.

“Seventy two hours after this happened, thousands of people who are amateurs at this or brand new to it have mapped something like 30,000 100,000 buildings,” says Girardot.

The group is looking for more volunteers. It’s possible to learn what to do–and start helping–in less than an hour. “If you can use a computer and mouse, after 45 minutes, you can really be contributing data that’s literally saving lives,” he says.