It is possible to reach the Maelstrom through a Recall potion or through failing to use teleport magic while unstable.
The landscape of the Maelstrom is constantly shifting and twisting. You may often get "stuck" between walls. Simply rest until they disappear. As you explore, you can expect to be randomly teleported. The Maelstrom cannot be mapped and there is no end to it.
High level monsters wander the Maelstrom in search of their own escape. Prismatic Masses are unique to this area.