While most MOBAs are all about spreading out your team across the map's several lanes in an attempt to get a level advantage over your opponent, many players enjoy single-lane maps as a casual diversion. And it's easy to understand why, as games on single-lane maps are pretty much just one, long 5v5 team fight.
Until now, no single-lane map has been available for Heroes of the Storm, so players have taken to setting games on the regular maps with gentlemen's agreements to only fight in the middle lane. Blizzard wants to help those players out, and are doing so by adding a new, single-lane map in the next patch for the game.
The Lost Caverns is designed specifically for huge team fights. Unlike the other maps in the game, there are no map objectives and only one lane. Scattered throughout the map are bushes and healing orbs in strategic locations letting players break off to recover or set up ambushes.
The Lost Caverns map won't be available in the regular Quick Match or Ranked Match modes. Players will instead have to setup custom games and then advertise them in chat. The official website has instructions about that, and more details about the map in general.
Lost Caverns will be added to Heroes of the Storm in an upcoming patch. No word on when, exactly.