Throughout the latter half of Path of Exile, you will find fissures (called Abysses) which open as you approach them, and monsters will spill out from below. You can pursue these cracks as they form, and they will lead to difficult encounters with great rewards.
Time is an important element when following an Abyss. Each Abyss you encounter will close after a short duration, but may spread further if you can slay enough monsters in time. If you're quick and a little bit lucky, an Abyss chest may appear at the end of the sequence of events. By slaying Abyss monsters you may be able to obtain items not found anywhere else:
Abyss jewels are special jewels with their own pool of mods. Unlike ordinary jewels, Abyss jewels have multiple tiers of many of their mods, as well as level requirements. They can also be socketed in a new type of item socket -- the Abyssal socket -- as well as in the passive skill tree. Abyssal sockets can be found on the Stygian Vise belt base type. At higher levels, some Abyss events will lead into subterranean areas with special bosses. The most dangerous of these bosses can drop special Abyss league-exclusive unique items with Abyssal sockets.