What's this hole on the ATR-72?
It is the ram air intake for the air conditioning system. There are two of them, one on each side.
Here is an image from the ATR-72 FCOM:

ATR-72 Air conditioning System, image from 737ng.co.uk
Details from the same document:

The air conditioning system is supplied by air processed through two packs which regulate air flow and temperature. The two packs are installed in the main landing gear fairings and operate automatically and independently.
The left pack supplies the cabin and cockpit air conditioned; the right pack supplies only the cabin air conditioned.

Also, as @Jonathan Walters points out, the intake is a good example of NACA inlet (or NACA scoop) and are found in a number of aircraft, for example Boeing 737.