A sub expert who raised the alarm about OceanGate's Titan tried to talk his pal Paul-Henri Nargeolet out of going on the doomed trip

• A submersible expert says he tried to talk Paul-Henri Nargeolet out of going on OceanGate's Titan. 
• Nargeolet, an explorer, was among the five people killed when the Titan imploded last month.
• "I couldn't have been more unequivocal in my condemnation of that thing," Patrick Lahey told Insider.

A longtime submersible expert who previously raised the alarm about OceanGate's doomed Titan sub, says it's "deeply troubling" that he couldn't talk his pal, explorer Paul-Henri Nargeolet, out of going on the doomed voyage to the Titanic shipwreck last month. 

"I couldn't have been more unequivocal in my condemnation of that thing," Patrick Lahey, the CEO and co-founder of Triton Submarines, told Insider during an interview on Tuesday, referring to the Titan.  

Lahey — whose Florida-based company designs and manufactures submersibles — said he told Nargeolet multiple times about his "grave concern for his personal safety and for anyone else" who planned to take a dive on the OceanGate vessel. 

Nargeolet, known as "Mr. Titanic" for having visited the wreck site of the iconic passenger line more than 35 times, was among the five people who died aboard the Titan when it imploded last month on an expedition to the Titanic. 

OceanGate's CEO Stockton Rush was also killed in the deep-sea disaster in the depths of the North Atlantic Ocean. 

Lahey called Nargeolet a "dear friend" who he had known for more than two decades and had sailed around the world with. 

Lahey said Nargeolet may have been "convinced by others" that journeying on OceanGate's unconventional carbon fiber and titanium sub "was maybe risky, but not that risky."

"But in point of fact, it was incredibly risky, as we all know ... [it] was ultimately the worst decision he ever made," said Lahey. 

Lahey said that he thinks Nargeolet, with his experience as a veteran deep-sea explorer, felt like "he could help" the other passengers on board the Titan if they found themselves in an emergency situation. 

In 2018, Lahey was among a slew of industry leaders who signed onto a letter sent to OceanGate expressing safety concerns about the Titan. 

"My biggest concern was that hull would collapse — it would collapse catastrophically and unexpectedly," said Lahey, who called the deadly disaster an "avoidable tragedy."

Lahey added, "You can't take people in a pressure boundary that is capricious, in a hull that's made of a material that is weakening and degrading in its strength over time."

An expert previously told Insider that photos of the Titan sub wreckage suggest that the most likely explanation for the vessel's June 18 implosion was that the hull collapsed under the immense pressure of the ocean.

The Titan, Lahey said, "was not safe for human occupants."