Frustrated by a lack of progress in official investigations into the disappearance of the Malaysia Airlines Boeing 777, victims' families combed beaches in Madagascar in 2016.

The final search vessel has left to make one final sweep of the deep sea to try and solve the greatest aviation mystery of all time, the disappearance of Malaysia Airlines Flight 370.

MV Fugro Equator left Fremantle on Monday night for the deep ocean ravines that might hold the remains of the Boeing 777 with 239 people on board that disappeared on March 8, 2014.

If the MH370 is not found in one of the deep ocean trenches then the search will be called off in January or February under an agreement between Australia, Malaysia and China.

REUTERS A wing piece found on Reunion Island in July last year was covered in barnacles and confirmed as belonging to MH370.

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It would resume if new information emerged.

"It has been an heroic undertaking but we have to prepare ourselves for the prospect that we may not find MH370 in the coming weeks, although we remain hopeful," Australian Transport Minister Darren Chester told the West Australian newspaper.

PAUL KANE/GETTY IMAGES The 'Fugro Equator' in Fremantle Harbour, Western Australia.

An autonomous underwater vehicle will be used that can dive six kilometres under the surface to explore about 200 trenches and canyons almost 3000km south-west of Perth that have not been investigated so far in the 120,000 square kilometre search zone.

Debris from MH370 have so far washed up in Mozambique, Tanzania, Madagascar, Reunion Island and Mauritius.

The plane veered off course while flying from Kuala Lumpur to Beijing, and flew south over the Indian Ocean for six hours before contact was lost, satellite data showed.