Erosion

Erosion is a multiplayer map that is featured in Halo 4 and Halo: The Master Chief Collection. It's intended use is to be used as a Forge canvas, and is one of four such maps to be designed and adapted to Forging.

Universe and Lore
With hundreds of human colonies now razed in the wake of the Human-Covenant War, the UNSC has dispatched large numbers of advanced pioneer groups to survey potential new worlds. The discovery of Eudemon X49-05 was considered lucky, but concerns remain about the moon’s structural integrity.

Layout
Erosion is set in an expansive cave system, including a large watery area inside. Part of the map serves as a Grifball arena. A human facility of unknown function resides here. It has multiple path ways, caves, and other areas of the map that can be largely Forged and played on.

Variant: Ascent
Ascent is a map variant of Erosion that is featured in  Halo 4 and Halo: The Master Chief Collection. It serves as Erosion's default Forge map, intended for Big Team Battle play. However, the map never featured in matchmaking.

Variant: Grifball Court
Grifball Court is a map variant of Erosion that is featured in  Halo 4 and Halo: The Master Chief Collection

As the name suggest, this map variant is only used in Grifball.

Trivia

 * It is possible to enter the inside of the "Rock Seastack" object on Erosion. This is not possible on other maps.
 * Erosion's pallet has a drastic difference in texture from its fellow Forge Maps. Rather than being a clean white in color, Erosion's objects have a rusted and dirty texture, which is in context with the map.
 * Erosion is often criticized for its uneven and sloped terrain that makes it difficult to Forge on.