More than 1 million people downloaded walkie-talkie app Zello in just a day as Hurricane Irma hit the Caribbean, and it's now topped Apple's app store chart in the U.S.
People are using it to talk to each other during Hurricane Irma, which is currently heading away from Puerto Rico towards the Dominican Republic and Haiti.
It's beating YouTube, Facebook and Snapchat right now in the chart, and reached 100 million users in February.
"We have seen a large number of people signing up for Zello in preparation for Hurricane Irma. Over 1 million people have joined in the last day, with most coming from Puerto Rico and Florida," said founder and chief technology officer Alexey Gavrilov in a blog post Wednesday.
Launched as Zello in 2012, the walkie-talkie app was ranked at number one on Apple's iTunes Chart on Thursday morning with people using it to communicate during Hurricane Harvey, which hit Texas late last month, its chief executive Bill Moore stated on LinkedIn on Wednesday.
Zello is different to messaging apps because people don't need to dial a phone number, or send a text. Instead they touch and hold an "echo" button to speak to each other live, and groups of up to 2,000 people can be created.
While it was launched as Zello five years ago out of Austin, Texas, it actually began life back in 2007 in Russia as Loudtalks, an app for Windows PC where people could talk to each other by holding down a "hot key".
After its official launch in San Francisco later that year, it was slow going, with the app picking up about 10,000 registered users in its first three months, according to a blog post on the Zello site.
But while it has 400,000 active users in Russia, the country blocked the app in April because it does not comply with laws governing "information distribution brokers." This means that Zello would have to provide user information to the government, something that Moore said it wouldn't be "able to meet or willing to comply with," even if it could.