The Howling Abyss is used for "ARAM" (All Random All Mid) matches, and is five vs. five.  The difference between the Abyss and the other maps is that there is only a single narrow lane of Turrets and an Inhibitor, and no neutral jungle area. Thus, rather than skirmishes and hidden movement, the Abyss focuses exclusively on large team-fights in the sole middle lane. Players cannot return to their allied base to replenish health and mana or purchase items unless they have been killed.  ARAM was launched as an official mode in September 2013.