Which trench is described as the world's only subzero hadal zone?

The South Sandwich Trench is described as the world's only subzero hadal zone. Its southern section extends below 60° south latitude into the Southern Ocean, where the deepest point is known as Factorian Deep.

Factorian Deep reaches approximately 7,434 metres and was surveyed during the Five Deeps Expedition in February 2019. Victor Vescovo piloted DSV Limiting Factor to the bottom on 4 February 2019, making the first crewed descent to the deepest point of the Southern Ocean. The name Factorian refers to the submersible that made the dive.

The trench itself is much deeper than Factorian Deep: its maximum depth is listed at about 8,266 metres. It formed through subduction, as the southern part of the South American Plate descends beneath the small South Sandwich Plate, creating the volcanic South Sandwich island arc. A common mix-up is treating the trench's 8,266-metre maximum as the depth of the subzero Southern Ocean section; the latter is the shallower Factorian Deep at about 7,434 metres.