The Underport is a location featured in Jak II. It is a vast network of ancient underwater passages located beneath the Port of Haven City, long-ago abandoned since the time of the city's founding. The area was visited by Jak and Daxter during the missions "Find Sig in Underport" and "Escort Sig in Underport", during which they had to use the Titan Suit to maneuver through the ruins of the Underport submersed in water and later reach Sig in the remnants of the still-in-tact Underport. Once they found Sig, they witnessed him battling off some metal heads that were flooding the old doors beyond the city wall. Sig had unknowingly let the metal heads into the city using the ruby key under the direction of crime lord Krew.
The Underport appears to be mostly constructed of stone and metal, with some areas seemingly ruined and overrun with earth. It is segmented by air locks, stone doorways, and large underwater buildings, littered with mines, metal heads, and ruined monoliths. Beyond the ruins, however, is an area still in-tact with large observatory windows, old rooms, corridors, traps and platforming challenges. The entire is possibly reminiscent of a former civilization.
The Underport is divided into two main parts; the ruined area and the ancient passageways. The ruined area is almost entirely submersed under water and is only ever traversed with the Titan Suit. This area is home to the only squid head metal heads, and also presents hazards such as mines and drowning (however, to counter the latter, various air vents are fixated throughout the area, allowing the Titan Suit to refill its oxygen reservoir). The ruined area is constructed mostly of stonework now sunken into the depths of the ocean floor, evidently comprised of ruined buildings, housing, and various monoliths. However, there is still some functioning technology in this area, such as elevator platforms, auto-seeking mines, the aforementioned air vents, and two air locks.
The ancient passageways are accessed following an air lock which drains the water from the area previous and allows the visiting of networks still remaining in-tact. This area is secure from the water, fortified by heavy stone and steel construction. Aside from the few rooms guarded by large stone walls, the area is traversed via floating, stationary, and moving platforms, impeded by various defenses such as electrical gates. The final area in particular, most rich in platforming challenges, is where a giant metal-pede resides, which both Jak and Sig had to outrun (however, Sig was later taken down by the massive metal-pede, but somehow survived). The area can then be exited via an elevator connected to a small maintenance station at the far west side of the Port's walkway.