The ocean trench that reaches the Horizon Deep, the deepest point in the Southern Hemisphere, is the Tonga Trench.
The Tonga Trench runs through the southwestern Pacific where the Pacific Plate is being forced beneath the Tonga microplate. This subduction zone helps make the trench exceptionally deep and is associated with intense earthquakes and volcanic activity across the Tonga region. It is also the site of the fastest-known tectonic plate motion on Earth.
Its deepest point, Horizon Deep, is about 10,823 metres below sea level. The name comes from the research vessel Horizon, whose crew located the deep in December 1952. In June 2019, explorer Victor Vescovo made the first crewed descent to Horizon Deep and direct pressure measurements confirmed its remarkable depth.
The Tonga Trench is the deepest trench in the Southern Hemisphere and the second-deepest known point on Earth. The Mariana Trench is deeper overall, reaching Challenger Deep in the Northern Hemisphere. The nearby Kermadec Trench is also very deep, but it does not contain Horizon Deep.