The Tonga Trench is 1,375 kilometres long and 10,882 metres deep at its lowest point — the Horizon Deep — making it the second-deepest spot in the ocean after the Mariana. It is the deepest place in the Southern Hemisphere. For most of human history, the hadal zone below 6,000 metres was assumed to be barren.

It is not. Expeditions over the past several years have confirmed thriving ecosystems at extreme depth: scavenging amphipods that descend below 10,000 metres, snailfish, large bigfin squid, communities sustained not by sunlight but by what falls from above and by chemosynthesis from microbes at the seafloor. A recent peer-reviewed study expanded the known scavenging amphipod fauna of the Tonga Trench to ten species, with up to twenty more believed to be undescribed across all previous South Pacific trench studies.

In January 2025, researchers from the Minderoo-UWA Deep-Sea Research Centre and partners deployed a baited camera at 1,400 metres on the eastern side of the trench. A mature female Pacific sleeper shark — estimated at 3.6 metres long — approached the bait, and the camera caught footage from inside her mouth. Pacific sleeper sharks are exceptionally rare to observe in the Southern Hemisphere; their life history is so little understood that researchers are using radiocarbon dating to estimate how long they might live, with growth rates believed to be comparable to the Greenland shark — which can exceed four hundred years.

The Tonga Trench sits along the path of the Antarctic Bottom Water, a deep ocean current that moves cold water from the Southern Ocean into the equatorial and North Pacific. It is, in other words, a major pipe in the global ocean circulation system.

The expeditions continue. Submersible technology, AI-assisted species identification, and the Ocean Census project’s aim of naming thousands of new ocean species by 2030 are all accelerating discovery. Each dive into the hadal zone has so far returned with more life than predicted.

Calls are growing for protected zones in the deep ocean before commercial deep-sea mining — particularly in the Clarion-Clipperton Zone, where twenty-four new amphipod species were described in 2026 alone — moves from exploration to extraction. The trenches were assumed lifeless for so long that the legal framework to protect them barely exists.

The work of describing what lives down there is, in a real sense, racing the work of figuring out how to mine it.