The catacombs, or more specifically known as the catacombs subrails, are vast planetary structures that were used by Mar and the Precursors to travel deep underground. Jak used them in order to access both the eco mine and the Precursor core in Jak 3. It is unknown if there were more or where they led, but they appeared to have been built wherever they were rendered necessary. They consist of large underground tunnels constructed with Precursor metal plates, cables, shields, lighted structures, and various defenses (such as rotating laser beams and small automatic turrets), appearing to possibly remain somewhat unfinished, having gaps and/or a complete lack of structure in some parts. They are traversed with catacombs rail riders.
The first time Jak visited the subrails was for the mission "Travel through catacomb subrails", where he accessed them via the Monk Temple in the Wasteland to get to the eco mine in Haven City. He visited them a last time from an entrance under the Haven Palace in a high-speed race against Veger (who did not actually appear during the mission) to the Precursor core in order to activate the Planetary Defense System and meet the Precursors themselves.