The London Diving School has designed an underwater bicycle, and they're challenging diving fans to cycle a mile on it to raise money for Sport Relief.
It's not just any bicycle - it's an underwater turbo training cycle - which is a cross between a mountain bike and a turbo trainer that works in water.
But the contest isn't all about speed, as going too fast underwater can actually be bad for you. Divers will be judged on how much air they use, their cycling style and even their costume!
Leah's been finding out more.