Room 14D is a place in the complex that is full of pits. The room first appears in Backrooms - Pitfalls, when a team of Async researchers (consisting of camera operator Marvin E. Leigh, Mark Blume, George Levy, and Kim) discovered it on May 6th, 1990 during an expedition.
During this expedition, Mark is seen crossing the room and reaching a door on the other side. Upon entering, he calls Marvin to follow and bring the camera. Marvin trips and falls into one of the pits, and finds himself deeper in the complex with his team still above in Room 14D.
On May 25th, 1990, Async successfully finds Room 14D again with the use of a rover. Mark Blume and his team are dispatched to re-enter the room, and report back to Team A. After collecting data on the dimensions of the pits, Team A begins work assembling a walkway across the room, leaving the pit that Marvin fell through exposed.