Amazon has launched Amazon Underground, a service that offers Android apps which would normally cost the user money for free.

Amazon has partnered with developers of both premium apps (such as OfficeSuite Professional) and those funded by in-app purchases (the Angry Birds games) and in some cases even both (Goat Simulator). All apps will be offered at no cost to Underground users, with any purchasable content available for free.

In exchange, these developers will be paid $0.002, or £0.0013 in the UK, for each minute a user spends in their app. As a result, any apps that run constantly in the background – Amazon specifically names anti-virus or clock apps in its terms and conditions – arent included in the scheme.

So wheres the money coming from? Perhaps unsurprisingly, advertising. The Underground app itself will show interstitial ads when launched or when the user accesses an app, with all of these revenues going to Amazon.

If downloaded as an Android app, Underground also features the retailers full mCommerce store – another potential revenue source for Amazon.

Its worth noting, though, that because of its app store element, Underground isnt directly available from Google Play, instead requiring the user to manually install an .APK file.

The Underground-partnered eapps will also be offered on Amazons own app store – as used on the its Kindle Fire tablets – marked with an “Actually Free” banner.