Majlis al Jinn is the second largest cave chamber in the world. It is located in a remote area of the Selma Plateau at around 1600 meters altitude in The Sultanate of Oman. It was discovered in 1983 by Don Davidson, a geologist studying water resources in the Sultanate. Davidson presumably died some ten years later when he left Oman permanently and went hiking in the Andes. He rented a car, drove it to a trailhead, left a note on it saying where he was going, and was never seen again.
Access is through a hole in the ground with a rope. The inner height is between 120 and 150 metres. The cave floor 300 by 200 metres. To give an idea of scale, the Great pyramid of Giza of Egypt (the largest Eygptian pyramid) would almost exactly fit inside the cave chamber; that is, its base is approximately the area of the chamber floor, and its height is also similar.