Searchers are scouring the North Atlantic for a missing submersible that had been due to visit the wreckage of the Titanic.
Fears are growing for the five people onboard as the vessel has oxygen for up to 96 hours.
Here is what we know so far:
What happened?
The 6.5-meter (21-foot) craft began its descent on Sunday but lost contact with the surface less than two hours later, according to authorities.
Its operator, a company called OceanGate Expeditions, told AFP in a statement late Monday that "for some time, we have been unable to establish communications with one of our submersible exploration vehicles which is currently visiting the wreck site of the Titanic."
The company uses a submersible named Titan for its dives to the wreck, with seats priced at $250,000 apiece, according to its website.
Who is onboard?
One of the passengers has been identified as British businessman Hamish Harding, whose aviation firm had posted on social media about his expedition.
Harding, a 58-year-old aviator, space tourist, and chairman of Action Aviation, had posted Sunday on his Instagram account that he was proud to join OceanGate's Titanic mission.
Pakistani businessman Shahzada Dawood and his son Suleman Dawood are also on the craft.
"As of now, contact has been lost with their submersible craft and there is limited information available," their family said in a statement.
Shahzada Dawood is the vice-chairman of Karachi headquartered conglomerate Engro, which has investments in energy, agriculture, petrochemicals and telecommunications
Where did the vessel disappear?
Searchers have been scanning a remote area of the North Atlantic where the Titanic rests 650 kilometres (400 miles) off the coast of Newfoundland, Canada, and some 4,000 metres (13,000 feet) underwater.
What's the latest on the rescue?
The US Coast Guard says it launched two planes and its Canadian counterparts have sent a plane and a ship.
Time is a critical factor, for the vessel has a maximum of 96 hours' air for five people.
US Coast Guard Rear Admiral John Mauger told reporters in Boston on Monday that he believed it still had 70 or more hours remaining.
But with no reported sightings of the vessel or communication signals, his team halted its flights for the day.
It said search operations through the night would be led by the US National Guard and the mission's operator.
Why visit the Titanic?
The 46,000-tonne liner hit an iceberg and sank on its maiden voyage from England to New York in April 1912 with 2,224 passengers and crew on board.
More than 1,500 people died, some of who were tycoons and aristocrats.
The disaster became popularised as an example of hubris, for the ship had been touted as a miracle of the industrial age and unsinkable.
Some also saw in it discrimination, as the vast majority of passengers who died were in second or third class.
The wreck was located in 1985 by a US-French expedition, deepening the fascination in the catastrophe, fuelling a blockbuster 1997 movie and spawning lucrative but high-stakes underwater tourism.