"FireChat" sounds like a phony location-based sex line.

It's not, it's a messaging app for iOS.

We already have Whatsapp, Facebook messenger, Snapchat etc, what makes FireChat different?

You can chat "off the grid", even if there is no internet connection or mobile phone coverage.

How is that possible?

Instead of relying on a central server, it is based on peer-to-peer “mesh networking” and connects to nearby phones using Bluetooth and WiFi, with connectivity increasing as more people use it in an area.

Firechat lets you talk anonymously

Where might this be useful?

According to FireChat, "on the beach or in the subway, at a big game or a trade show, camping in the wild or at a concert, or even travelling abroad, simply fire up the app with a friend or two and find out who else is there."

Seriously though.

In Hong Kong mostly, where pro-democracy protesters are using it to communicate amid fears of network shutdowns.

It's also been used by Iraqis and Taiwanese students during their anti-Beijing Sunflower Movement.

Aside from not being reliant on the internet (which some governments restrict), it is more clandestine and less traceable.

How popular is FireChat?

Over 100,000 people downloaded it in 24 hours in Hong Kong over the weekend, with the CEO saying that numbers are "booming" and up to 33,000 people were using the app at the same time.

Will it catch on in the Western world?