This underwater "Ironman" jetpack lets you swim faster than Michael Phelps

How did Shulman feel jumping into the water for the first time?

“It was absolutely scary,” he said. “This was really putting your money where your mouth is. I’ve never built a project and then strapped it onto myself before.”

As their finished YouTube video attests, Shulman survived, and the project was a complete success. At top speed, the underwater jetpack clocked in at 6.25 mph, breaking Phelp’s record of 6 mph. The propellers were more powerful than expected, Shulman said, noting that at times it felt like they were “going to pull my arms off.”