Scientists will venture into the twilight zone in a never-before-seen part of the ocean to search for new forms of life.
The mission, called First Descent, will take scientists from the research organisation Nekton down to a depth of 300m (984ft), where sunlight barely penetrates yet thousands of undocumented species could flourish.
The team on the Ocean Zephyr vessel will make at least 50 dives in remote coral reefs of the Seychelles, first around Farquhar and then Aldabra in March.
Nekton scientists will survey the health of the coral in the area and assess the biodiversity of the reefs, in an area believed to be teeming with sharks.
The data will be used as a baseline in the rapidly changing environment of the Indian Ocean, where climate change, overfishing and plastic pollution are major threats to coral and the life it supports.