The Bottomless Canyon (無限渓谷 Mugen Keikoku, lit Infinite Chasm in the Japanese version) is a location in Fire Emblem Fates.
It is a massive geological feature that separates Hoshido in the east from Nohr in the west. As the name suggests, the canyon seemingly has no "bottom", thus anyone who falls into it never returns and presumably dies as a result.
In reality, the Bottomless Canyon is a portal to the world Valla resides in from the world that Hoshido and Nohr exist in. Although it is one among many, it is the easiest portal to enter. However, the portal opens once every few decades and the time to use it is limited. One must simply jump into the canyon to be transported to Valla.
The canyon's southern end terminates in the Hoshidan sea, the farthest reaches of the canyon to the north are never seen. The canyon is sandwiched between mountains running parallel to both sides for its entire length. The canyon and surrounding mountains take after Nohr's landscape, the earth houses little plant life, and is rocky and barren. The canyon itself is often beset by thunderstorms and is difficult to cross due to both size and weather conditions. This makes the canyon a great natural border between Hoshido and Nohr, and is primarily crossed by bridge. There's also a huge Lava Field on the Nohr side of the canyon.