From the Super Mario Wiki
The Coward's Chasm is a room inside of the Secret Mine in Luigi's Mansion: Dark Moon. The Coward's Chasm is accessed by taking the door on Floor B3 of the Shaft. The Coward's Chasm is a small room with a large mirror on the north wall. A small cavern can be seen on the south of one of these walls and there is a lantern attached to a pulley system. There is actually a pathway behind the wall to the north side. Inspecting the large ice wall on the left allows Luigi to go back in the Shaft. The door on the right leads into the Deep Hall.
During D-2: Hit Rock Bottom, Luigi needs to go through the Coward's Chasm to reach the Crystal Quarry. To reach the other side of the chasm, Luigi needs to go north and walk behind the wall that is being reflected by the mirror. During this mission and most missions after, Luigi can lower the lantern with the valve nearby to cause three gold bats to fly out of their cavern.