Which oceanic trench contains the Horizon Deep?

The Horizon Deep is contained within the Tonga Trench. Located in the southwestern Pacific, the Tonga Trench marks a subduction zone where the Pacific Plate sinks westward beneath the Tonga microplate and nearby parts of the Australian Plate.

At about 10,823 metres deep, Horizon Deep is the deepest point in the Southern Hemisphere and generally regarded as the second-deepest known point on Earth, after Challenger Deep in the Mariana Trench. It was identified in 1952 by researchers aboard the Scripps Institution of Oceanography vessel Horizon, which gave the feature its name.

The Tonga Trench is sometimes confused with the nearby Kermadec Trench, but they are distinct sections of the broader Tonga–Kermadec subduction system. Horizon Deep belongs specifically to the Tonga section. In 2019, explorer Victor Vescovo made the first crewed descent to the site during the Five Deeps Expedition.