Believe it or not, there's something more terrifying than deep sea diving and being stuck inside a submarine: deep sea diving and being stuck outside a submarine. 

These scuba divers off the coast of Hawaii experienced that exact fear, when a submarine appeared from the depths of the ocean directly beneath them. Fortunately, it appears to be an Atlantis tour submarine; a tourist submarine experience that lets passengers get an up-close view of marine life at shallow depths. Were it a military submarine, that would have been a very different story. 

"I had one of these sneak up on me diving in Cozumel last summer," @DustinM64 commented. "It definitely was a surprise to see it come out of the blue!"

The divers are also fortunate that there were no sonar pings from the submarine, as those can be deafeningly loud for divers and marine life around them. Just check out this terrifying clip from divers experiencing a sonar ping from a mysterious distant sub. 

Believe it or not, I think I'd rather stay inside a submarine.