After riding Atomos from Mah'habara, you will arrive here. If you activated the Waystone in Mah'habara, Mission 19 will be available. Also, if your ride Atomos back, you will unlock several new areas filled with nasty Monsters. (Though you're not really supposed to know that yet.) Note that the Treasure here is found in the second part of the area, past the Subterranean Lake.