(NewsNation) — British billionaire Hamish Harding is confirmed to be one of the five people aboard a submersible watercraft lost at sea.

Owned by OceanGate Expeditions, the Titan began its dive into the Atlantic Ocean on Sunday morning and hasn’t surfaced. The craft has five people aboard and was going to view the wreckage of the Titanic.

Jannicke Mikkelsen, a friend and colleague of Harding, says Harding is an “explorer at heart” and she hopes he is OK.

“This was yet another goal on his checklist of what he wanted to explore down here on Earth, and also in space,” Mikkelsen said Monday on “CUOMO.” “Titanic was one of the big high points that he wanted to achieve in his life.”

The U.S. Coast Guard and Canadian authorities initiated a search for the vessel Monday, a day after it lost contact with the research boat it was communicating with on the surface. There was a 96-hour oxygen supply for the crew of five when it launched, giving rescuers roughly until Thursday morning to locate them.

“My biggest fear is (if) they are trapped without the ability of receiving help. There is no one who can reach him at the bottom and — this is terrifying,” Mikkelsen said.

OceanGate’s expeditions to the Titanic wreck site include archaeologists and marine biologists. The company also brings people who pay to come along, known as “mission specialists.” They take turns operating sonar equipment and performing other tasks in the five-person submersible. The Coast Guard said Monday that there was one pilot and four “mission specialists” aboard.

One of the mission specialists is Harding, a billionaire adventurer who holds three Guinness World Records, including the longest duration at full ocean depth by a crewed vessel. In March 2021, he and ocean explorer Victor Vescovo dived to the lowest depth of the Mariana Trench. In June 2022, he went into space on Blue Origin’s New Shepard rocket.

The Titan is made of “titanium and filament wound carbon fiber” and has proven to “withstand the enormous pressures of the deep ocean,” OceanGate has stated of the vessel’s safety.

In a May 2021 court filing, OceanGate said the Titan had an “unparalleled safety feature” that assesses the integrity of the hull throughout every dive.

Mikkelsen said Harding would have known and accepted any risks associated with the dive.

“Every single expedition is dangerous, and that’s something that Hamish taught me. He’s the reason why I started my expedition career, as well,” Mikkelsen said. “Every expedition has the possibility of death, absolutely, and Hamish knows the risk of every single expedition he starts.”

Mikkelsen is thankful for the search efforts underway by U.S. and Canadian authorities that she hopes will yield positive results.

“Hamish is a close friend, his family are close friends, and Hamish is the reason why … I started my expedition career. I owe a lot to Hamish,” she said. “I hope he’s OK. I hope his crew members are OK and they resurface.”

The Associated Press contributed to this report.