From the Super Mario Wiki
The Canyon Stairs (Ravine Stairs in the British English version) is a room inside the Old Clockworks in Luigi's Mansion: Dark Moon. The Canyon Stairs is accessed by taking the northeast door inside of the Canyon Hall. The Canyon Stairs is an optional room and doesn't need to be explored during any mission, but it is the only way out of the basement of the Old Clockworks. This room is essentially a flight of stairs that leads to the Roundhouse. Blowing up the debris witht he robotic bomb makes Luigi receive a call from Professor E. Gadd thanking him for opening the way, but to not stray from the mission.
Starting from C-2: Underground Expedition and most missions after, Luigi can go to the Canyon Stairs and yank off a web with with a ruby in it.