On Apr. 28, 2025, deep-sea fish adapt to some of the most extreme conditions on Earth. A study, published in the journal Cell, researchers analyzed the DNA of 11 fishes, including snailfish, cusk-eels and lizardfish that live in the hadal zone — the region about 19,700 feet (6,000 meters) deep and below — to better understand how they evolved under such extreme conditions.

The researchers used crewed submarines and remotely operated vehicles to collect samples from about 3,900 to 25,300 feet (1,200 to 7,700 m) below the water’s surface, in the Mariana Trench in the Pacific and other trenches in the Indian Ocean.

Fish that survive in extreme deep-sea environments have developed the same genetic mutation despite evolving separately and at different times, researchers say.

The scientists also found industrial chemicals in fish and in the ground in the Mariana Trench, meaning human-made pollutants can reach some of the deepest environments on Earth.

Tracing the evolution of deep-sea fishes, the researchers’ analysis revealed that the eight lineages of fish species studied entered the deep-sea environment at different times: The earliest likely entered the deep sea in the early Cretaceous Period (about 145 million years ago), while others reached it during the Paleogene (66 million to 23 million years ago), and some species as recently as the Neogene period (23 million to 2.6 million years ago).

Despite different timelines for making the deep sea their home, all the fishes studied living below 9,800 feet (3,000 m) showed the same type of mutation in the Rtf1gene, which controls how DNA is coded and expressed.

This mutation occurred at least nine times across deep-sea fish lineages below 9,800 feet, study author Kun Wang, an ecologist at Northwestern Polytechnical University, told Live Science.