For example: An elderly victim screaming “help” as the waters rise in the aftermath of Hurricane Harvey.

“With voice, someone can communicate a ton of information in a way that text does not,” said Bill Moore, the chief executive of Zello, a free Internet “walkie-talkie” app. “In a few seconds of hearing your voice, I can guess what part of the country you’re from, if you’ve been drinking, what mood you’re in, whether you’re afraid or in distress.”

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“For that reason, voice becomes great for solving problems, and it demands attention from both sides in a way that texting does not,” he added. “Your brain is wired for voice.”

Therein lies the secret behind the proliferation of Zello, which has become the preferred mode of communication for organizations such as the “Cajun Navy,” an informal group of Louisiana boat owners who participated in this week’s search-and-rescue missions in Southeastern Texas.

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The app allows flood victims and rescuers to communicate instantly. It also allows both groups to post voice messages to specific channels that have been set up to aid people seeking assistance, such as “Texas Volunteer Rescue/Support” and “Harvey Animal Rescue” and the “CajunNavy,” which has nearly 25,000 users.

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Over the past week, Moore said, Zello usage has increased twentyfold. The number of user sessions increased 600 percent over the past week, with the amount of time users in the Houston area were on the app increasing to 22 minutes, Moore said.

The app is another powerful example of how social media is filling the humanitarian holes that local government is unable to plug, turning ordinary people into heroes and empowering desperate flood victims to reclaim their fate from the rising floodwaters. Flood victims have also turned to Facebook, Instagram and Twitter to reach private rescuers when public officials have been overwhelmed. But as messages on those platforms proliferated and became unwieldy to manage, the Cajun Navy began asking people to connect with rescuers using Zello exclusively.

***IF YOU NEED RESCUE OR WANT TO HELP RESCUE*** The best source of communication at this time is the Zello walkie talkie... Posted by Louisiana Cajun Navy on Monday, August 28, 2017

The app relies on cellphone data plans or WiFi, Moore said, but was designed to operate in areas where signals can be weak, such as those served by the outdated telecom technology known as 2G.

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“That’s why it’s so popular in disaster areas,” Moore said.

Rescuers consider the app a hybrid between talking and texting. Unlike a Twitter conversation, which can be difficult to follow, Zello allows groups of people to communicate simultaneously, which is advantageous for grass-roots rescue efforts and has led some to label the app “social radio.”

The app has 100 million users around the globe, Moore said, often in countries were government services struggle to meet demand, such as Egypt, South Africa, Venezuela, parts of the Middle East and now parts of the U.S. Gulf Coast.

“It’s really common in times of trouble for a rescue structure to emerge organically,” Moore said. “The Cajun Navy is using Zello to augment the emergency services and build a culture that is quick and effective.”

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There are limitations. The app doesn’t work if it can’t connect to a network, and users must be aware of its existence, an issue for any new product. That can be a problem for those who don’t own mobile phones or are unfamiliar with apps. Overwhelmingly, those are the same people who are most vulnerable during disasters — the poor and the elderly.

Still, the app offers a window into the role technology can play in a crisis.

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As the tragedy in southeast Texas unfolded this week, rescuers could be heard discussing all manner of chaos, from an elderly couple trapped on their roof to a fire at a chemical plant that threatened to contaminate the surrounding floodwaters.

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“I have a diabetes patient and a sick baby,” a woman could be heard saying Wednesday, before offering her address. “I need a boat rescue.”

“I have a 102-year-old male in Port Arthur taking on quite a bit of water, and he cannot swim,” another woman said urgently moments later. “He is in a panic.”

“There’s an elderly lady, 61, rising water,” another user reported. “She needs help.”

The app was also being used to help rescuers coordinate among one another. Between desperate calls for help from all over the region Wednesday afternoon, boaters checked in to confirm particular victims had been rescued, trade information about inaccessible areas and offer details about a staging area where boaters could grab a bite to eat and talk to the media. Moments later, a request arrived for boaters to patrol that night with sheriff’s deputies attempting to keep looters at bay. People seeking to volunteer their experience, such as individuals with boats and fast-water rescuing experience, also left messages offering their service.

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When Zello channels experience high traffic, victims are encouraged to write down crucial information about their location and send a picture to channels that rescuers can view, which happened constantly Wednesday.

“Is anyone in the southwest area of Houston, by loop 610 and highway 59,” a man could be heard saying. “I have an address to submit to you guys. It’s not for me, it’s for someone on my Periscope page.”

Nearly 170 miles away in Austin, where Zello is based, Zello’s chief executive was listening as well.

Moore said he’s been spending several hours a day listening to Zello traffic in his office while working. He hears tragedy, of course, but also triumph.

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“It’s really fascinating to listen to,” he said. “You find yourself wanting to cry when someone is about to drown and then feeling thrilled when someone has been rescued.”

“The app feels like a shared experience,” he added.