Oxygen onboard the Titan submersible will run out at 12.18 Irish time today, the US Coastguard estimates.
The OceanGate expedition to see the wreck of the Titanic lost communication on Sunday and a rescue operation was launched.
Officials say they are "optimistic and hopeful" with 10 more ships and submarines due to join the operation today.
However, the captain in charge of the search said, “We don't know where they are, to be frank with you."
The search area now covers an area roughly the size of the US State of Connecticut and involves camera equipment scanning the sea floor.
At various points, banging noises have been heard; however, former US Navy nuclear submarine commander David Marquet has cautioned it could be something else.
“I don’t think the noise is them, it could just be natural sounds,” he told the BBC.
“We’re hearing noises and more ships are coming into the area, and then we’re hearing more noises and I don’t think that’s a coincidence."
Safety concerns were raised about the Titan in 2018; the Marine Technology Society said it had “concern regarding the development of Titan and the planned Titanic Expeditions” and warned against the “current experimental approach adopted by OceanGate”.
It also called for the vessel to be independently assessed by.
A Titanic story
Onboard the submersible are the businessman Hamish Harding, businessman Shahzada Dawood and his son Suleman, explorer Paul-Henry Nargeolet and OceanGate CEO Stockton Rush.
Mr Rush’s wife, Wendy Rush, is the great-great granddaughter of two Titanic passengers - Isidor and Ida Straus.
Titanic survivors. 1912The Strauses were first class passengers and co-owned the famous New York department store Macey’s.
Survivors recall Isidor refused to board a lifeboat until all women and children had got on board and Ida then refused to leave without him.
The pair were seen embracing as the ship went down.
Main image: The Oceangate submersible "Titan". Picture by: Alamy.com
