Last Wednesday, Hurricane Michael slammed into the Florida Panhandle at 155 miles per hour, flattening neighborhoods, turning subdivisions into rubble, and plunging the coast into darkness. On Friday, Trevor Lewis packed up two trucks with crowbars, chainsaws, sledgehammers, ropes, walkie talkies, and five other guys from Cocoa Beach, where he lives on the east side of the state. As night fell they began the drive up into the worst of the wreckage. By 4 am on Saturday they were responding to their first call for help.

Lewis leads a self-funded search-and-rescue unit made up of off-duty police officers, firefighters, and EMTs. They call themselves Salty Water Rescue Services, and most of them have special emergency training on the high-speed powerboat racing circuit off Cocoa Beach. They’re one of hundreds of volunteer crews that showed up post-Michael to help first responders overwhelmed by 911 calls.

They used as their guide a service called Crowdsource Rescue, or CSR, which showed on a map individuals who might need help. On one of Salty Water’s visits, the crew met a woman whose house had a gas leak, so Lewis called local authorities. “It’s an eerie feeling to dial that number thinking someone’s going to come and it goes straight to a busy signal,” he says. The woman’s family had used CSR to request a wellness check on their relative.

The idea behind CSR started out simple: collect calls for help posted on social media, geolocate them, and route volunteers to the distressed parties. Basically, Uber for emergencies. It was a simple enough concept that a pair of developers named Matthew Marchetti and Nate Larson hacked it together in about six soggy hours in Houston last August while Hurricane Harvey howled outside. They expected it to help out a few families in their rapidly flooding neighborhood. By the time the storm was over, Marchetti says at least 25,000 people had been reached using the web service.

Turns out, armies of spontaneous samaritans can get a lot more done if technology tells them where to go. “The volunteers are going to show up no matter what,” says Marchetti. “We’re just trying to empower them to find more people safely and more effectively.”

What began as a one-off charitable coding sprint has since evolved into a five-person emergency volunteer mission control center. In their day jobs at a real estate company, Marchetti and Larson built maps. And that’s basically what the first version of CSR was. But as more hurricanes rocked the US in quick succession in late 2017—first Irma, then Maria—the pair piled on new features as problems arose in real-time. “We would change something on the site while there were 60,000 people using it, and if they didn’t like it, they’d let us know,” says Marchetti. “We had no master plan. It was all just reactionary.”

In January, when the hurricane season had ended, they found time for a more thoughtful redesign. The latest version, released two weeks before Florence hit, includes a mobile app and new safeguards. It allows users to tag hazards such as downed power lines, washed-out roads, and fast-moving water. It also gives CSR the flexibility to block off any areas that emergency management officials have declared dangerous for civilians, so that volunteers without proper training can’t see aid requests in those areas. Those precautions are to prevent volunteers from winding up needing rescuing themselves.

That’s less of a concern for volunteers like Lewis’s Salty Water crew, who used CSR for the first time during Florence, but who are no strangers to treacherous waters. After that September storm swept through, seven of their guys drove to North Carolina with a flat-bottom boat and two tuned-up jet skis with rescue sleds on the back. Over the course of a few sleepless days they checked in on nearly 100 people, delivering supplies, relaying messages to loved ones, and helping the unluckiest few to safety, including about a dozen shivering pets.