You’re thinking of the Titan Submarine Implosion. Stockton Rush created a submersible to take rich tourists down to see the wreckage of Titanic. He made of carbon fiber, thinking that would be particularly resistant to the crush damage, and ignored experts and warning signs that it was already breaking during test runs.

Behind the Bastards did a two-parter on Stockton Rush that’s worth the time, if you want a deep dive.