Portland Rock is a small island within the boundaries of Portland Beach, Liberty City. The rock also contains a lighthouse. The Portland Rock appears in the final mission of Grand Theft Auto: Liberty City Stories, The Sicilian Gambit. The rock is unreachable in Grand Theft Auto III. If the player manages to reach it, the player will be forced to slide down the island. The stairs leading to the rock was probably cut off from the water by a storm or was not maintained.
One way to get up to the lighthouse in the PC version of GTA 3 is to take a boat to the island, turn your back towards the island and then jump up the island backwards.