A video shows the cramped conditions inside the Titan submersible that went missing during a dive to the Titanic shipwreck.
Rescue teams are in a race against time to find the vessel, which had only 96 hours of oxygen on board when started its dive in the early hours of Sunday morning local time.
The five passengers trapped on board are also contending with cramped conditions on a vessel that has no seats and is operated with what looks like a PlayStation controller.
Among those on board are British billionaire Hamish Harding, 58, and British-based Pakistani businessman Shahzada Dawood, 48, and his son, Suleman, 19.
French explorer Paul-Henri Nargeolet, who works for OceanGate, is also thought to be on board, as is OceanGate founder and chief executive Stockton Rush.
Inside the Titan submersible
OceanGate Expeditions charges individuals $250,000 (£195,000) for an eight-day trip, which includes a dive to the wreck of the Titanic.
According to the OceanGate website, the submersible is made from carbon fibre and titanium, weighs more than 10,000kg and can reach depths of 13,100ft.
A video posted on YouTube by OceanGate gives a sense of just how cramped it is inside the submersible.
The Titan submersible is equipped with its own toilet. (CBC)
OceanGate founder Stockton Rush, who is on board the missing Titan submersible, showing its features. (CBC)
There are no seats inside the Titan submersible. (PA)
There is room for five people – one pilot and four crew – but there are no seats, and those on board must sit close together.
Read more: Missing tourist sub locked with bolts from outside
The Titan is 22ft (670cm) long, 9.2ft (280cm) wide and 8.3ft (250cm) high, and can reach a speed of 3 knots.
There is enough emergency oxygen available on board to last 96 hours.
Watch: Inside the missing submersible that went missing during Titanic voyage
Titan is the only deep diving submersible with a toilet. Similar to a camping toilet, this is accessed in the forward dome and comes with a privacy screen.
There are three screens inside the submersible, one for controls, one for sonar and one for imagery. The inside of the submersible has just one on-off button on the interior wall, while the craft is run by a device that bears a similarity to a Sony PlayStation controller.
OceanGate founder Stockton Rush, who is on board the missing submersible, explained in an episode of the BBC’s The Travel Show last year how it works.
The features of the missing Titan submersible. (PA)
The conditions inside the Titan submersible are cramped. (OceanGate)
“I like to tell people this is not your grandfather’s submarine,” he said.
The Logitech controller used to pilot the Titan submersible. (CBC)
“It’s got one button and that’s it. We run the sub with this game controller. It’s made by Logitech, but it’s basically a Sony PlayStation style controller.
“If you want to go forward you press forward. If you want to go back you press back. You want to turn to the left it’s like that, you want to turn to the right you turn to the right.”
How does it work?
According to OceanGate, the sub is “a combination of groundbreaking engineering and off-the-shelf technology”.
Dr Robert Blasiak, an ocean researcher at the Stockholm Resilience Centre at Stockholm University, told BBC Radio 4’s Today programme: “This submersible is designed to go down to certain depths. And this Titan sub, according to its specifications, is able to go down to depths of 4,000m.
“The Titanic is resting on the sea bed at a depth of about 3,800m, so it’s within the specifications.
The Titan submersible, pictured launching from its docking platform. (OceanGate, Alamy)
The Titan submersible vessel, pictured on the surface in this file photo. (PA)
“But we have to understand that the forces at that part of the ocean are tough to even wrap your head around. It’s about 380 atmospheres of pressure, so 380 times the pressure you feel on the surface walking around. That’s about 200 times the pressure that’s in a car tyre, so imagine that all around you. That’s why you need a submersible with really thick walls.”
On the Frequently Asked Questions section of its website on 30 May, OceanGate said the Titan communicates with the crew on the surface via text messages sent via a USBL (ultra-short baseline) acoustic system.
The view for passengers on board the Titan submersible. (OceanGate)
The OceanGate submersible Titan went missing on Sunday evening. (Alamy, OceanGate)
It said the submersible is required to communicate with those on the surface every 15 minutes or more frequently if needed.
The same system is used to track the position of the submersible.
Anyone who pays the $250,000 fee to go on board the Titan becomes a “mission specialist”, and is tasked with gathering data during the expedition.
The website states: “You do not need any previous maritime experience or a specific skill set in order to join. It is helpful to have a sturdy pair of sea legs!”
– Everything we know so far about the missing Titan submersible (Evening Standard, 4 mins)
Watch: Search for submersible tourist vessel near Titanic wreck