The Broken Mines are the final area of Torchlight II, accessible in Act IV.
The hub is Minehead, where all the usual vendors are available.
The dungeon resembles Torchlight I, when you descend with every next level. On levels 1-3 you meet only dwarven enemies. On level 4, which is a small maze of teleporting beacons, you finally meet The Dark Alchemist. The fight has three consecutively harder stages. Prepare for running and using ranged attacks, as the opponent likes to teleport himself and use a suppressing fire beam attack during the 2nd and 3rd stage.
On level 5 there is a waypoint, from this level on the Netherim start to dominate among the enemy crew. On level 8 you finally get to meet The Netherlord.