Hydro is a unique map in that it has six areas with each their control point, but only two are accessable at any given time. The two teams start with control of three of the areas each and each round two random areas are active with various routes opening up depending on the active areas. If a team captures five of the six areas they are given a chance to capture the other teams base, with their own control point being locked. If the final point is defended, an area is given back to the defenders and the game continues.
Hydro will often end in stalemates and isn't very popular with the playerbase, which might explain why no other Territorial Control maps have been made.