Hawaiian Trough

The Hawaiian Trough, also known as the Hawaiian Deep, is a depression of the sea floor surrounding the Hawaiian Islands, where the massive weight of the island chain downwarps the oceanic lithosphere; surrounding the islands like a moat, it is roughly 5,500 metres (18,000 ft) deep. However, in accordance with the principle of isostasy, the sinking of the lithosphere is balanced by a corresponding rise beyond it, known as the Hawaiian Arch. The arch is lifted 200 metres (660 ft) above the local seafloor, and has tilted coral reef terraces that used to lie flat.

The Big Island itself is still subsiding, at a rate of about 2.5 millimetres (0.1 in) per year.