Japanese researchers reported on Thursday that many of the creatures they dredged up are one-celled animals called foraminifera.
What surprised the researchers, however, was not just that these animals do not normally live so deep in the ocean but that all their known relatives have hard shells.
The little organisms may have slowly adapted to the dark and high pressure found in the trenches 10km under the ocean, the researchers report in Friday's issue of the journal Science.
Yuko Todo of Shizuoka University in Japan and colleagues used the KAIKO Remote Operated Vehicle to sample the very deep trenches of the western Pacific, which they said reached their present depths six to nine million years ago.
"The lineage to which the new soft-walled foraminifera belong includes the only species to have invaded fresh water and land.
"Analysis of the new organisms' DNA suggests they represent a primitive form of organism dating back to Precambrian times from which more complex multi-chambered organisms evolved."