Summary
Backwash is a multiplayer level in Halo 2, similiar to the campaign level on Halo 1 called 343 Guilty Spark. It is a large rainforest set on Delta Halo. You can tell it is Delta Halo, not Alpha Halo, as 2401 Penitent Tangent can be seen floating around and humming to himself.
It is the only multiplayer map which contains the Sentinel Beam.