Deep sea explorers Nekton have arrived at their first location for a series of dives into the twilight zone.
The mothership, the Ocean Zephyr, arrived off the coast of Alphonse, a remote island of the Seychelles, just before 6am local time (2am GMT).
The atoll sits on a “sea mount” – an underwater mountain that rises 4,000m (13,123ft) from the ocean floor.
The team will practise the precarious launch of the £2m subs.
During the first test, near the capital, Victoria, the fibreglass machines swung alarmingly as they were hoisted by a crane into the water.
Once the team has perfected safe launch and recovery they’ll run a series of tests underwater.
Only then can the science begin.
The team will descend to 300m (984ft) – well into the ocean twilight zone – to search for new species and do a “health check” of the ecosystem.
Research earlier this week showed the Indian Ocean is particularly vulnerable to “marine heatwaves”, caused largely by climate change, that damage or kill coral.
But scientists just don’t know what is happening in the deeper waters, inaccessible to scuba divers.
Randy Holt, one of the pilots from Global Sub Dive, is looking forward to diving in an area that has never been explored before. He has done more than 350 dives to depths of up to 1000m (3281ft).
He said: “It’s amazing to be down with life you’ve never seen before”.
“It’s peaceful. If it is a flat bottom we can land there.
“I enjoy turning the lights off, turning down some of the noisy systems and sitting in the darkness, in the quiet, and watching what happens.
“Bioluminescence (light generated by creatures) will come by so you see sparkles, and you can hear the shrimp crackling.”
Engineers spent the 36-hour voyage from the main Seychelles island of Mahe fixing specimen collection boxes to the exterior of the subs and ensuring a watertight seal in the wiring that carries power and control signals.
Robert Carmichael, who is also a sub pilot, said a leak could prove catastrophic.
He said: “Seawater wreaks havoc on all the electronics.
“Everything has to work to pull off the science and the media mission.
“One little thing could spoil the whole apple cart, so we have to be meticulous.
“Then of course there’s the life support aspect.”
At 300m (984ft), the pressure is immense, equivalent to a jumbo jet pressing down on your head.
Just 8cm of acrylic plastic separate the occupants of the sub from oblivion.
The manufacturers Triton say although 305m (1,000ft) is the maximum operating depth, the sub is built to withstand pressures down to almost 400m (1312ft).
:: Sky News will broadcast live from 300m down in the Indian Ocean on 18,19 and 20 March. The series will examine the impact of climate change and plastic pollution. It includes the deepest-ever live news programme from submersibles.
:: Sky’s Ocean Rescue campaign encourages people to reduce their single-use plastics. You can find out more about the campaign and how to get involved at www.skyoceanrescue.com
(c) Sky News 2019: Deep Ocean Live: Subs practice launch for first twilight zone dives