Fig. 2: Representative fauna of cold-seep sites in the Kuril–Kamchatka Trench and western Aleutian Trench.

a, Free-moving polychaetes Macellicephaloides grandicirra (white; reaching 6.5 cm in size) navigate among dense colonies of frenulate siboglinids, with tubes 20–30 cm in length and approximately 1 mm in diameter, at 9,532 m at The Deepest. b, Clusters of frenulate siboglinids extending red haemoglobin-filled tentacles, with small Gastropoda (white spots) on tops of the tubes near the tentacles, at 9,320 m at Wintersweet Valley. c, Tightly packed frenulate siboglinids are home to abundant free-moving polychaetes M. grandicirra (white) at 9,332 m at Cotton Field. d, Dense aggregation of vesicomyid bivalves A. phaseoliformis (reaching 23 cm in size) in the sediment, with approximately 6–8 cm of valves exposed and often hosting Actiniaria, at 5,743 m at Clam Bed. e, Tube-dwelling polychaetes Anobothrus sp. and Actiniaria are dominant at 6,870 m at Aleutian Deepest, with spots of white microbial mats. f, Dense aggregation of vesicomyid bivalves I. fossajaponicum (reaching 3 cm in size) associated with black sediments and accompanied by tube-dwelling polychaetes Anobothrus sp. at 6,928 m at Aleutian Deepest. g, Dark blue muds surrounded by clusters of frenulate siboglinids, mark methane seeps at 6,800 m at Blue Marsh. h, Large patches of white, snow-like microbial mats stretch tens of metres, accompanied by frenulate siboglinid tubeworms at 6,700 m at Icy River. The images were taken by the manned submersible equipped with a high-definition camera system. The name of each cold seep indicated in the lower left corner. The distance between laser beams is 10 cm. An expanded showcase of cold-seep fauna is given in Supplementary Video 1.