The Catacombs are vast underground Precursor structures that were used in the original creation of the world; they were locations in Jak 3.
Jak and Daxter visit the catacombs twice in Jak 3, once to travel to the Eco Mine, the second time to reach the Precursor Core in pursuit of Count Veger in a very high speed race. Both times they use the Catacombs Car.
The Catacombs contain access to the Precursor Core and Mar's Eco Mine. They are small high-tech rails which were traversed by a hovering car.