What is the second deepest ocean trench after the Mariana Trench?

The second deepest ocean trench after the Mariana Trench is the Tonga Trench.

The Tonga Trench runs through the southwestern Pacific Ocean, east of the Tonga island group. Its deepest point, called Horizon Deep, is approximately 10,800 meters below sea level. That makes it the deepest point in the Southern Hemisphere and the second deepest known point on Earth after Challenger Deep.

The trench formed at a major subduction zone where the Pacific Plate plunges westward beneath the Tonga Plate. This boundary is among the fastest-moving plate-tectonic zones on Earth, helping explain the trench's exceptional depth and intense earthquake activity.

The Tonga Trench is sometimes confused with the nearby Kermadec Trench because both belong to the broader Tonga–Kermadec subduction system. They are distinct named sections, however: the Tonga Trench is the deeper southern section, while the Kermadec Trench is shallower. Horizon Deep was named after the research vessel Horizon, whose crew identified it in 1952.