Diving in lakes and oceans takes both skill and practice to ensure everyone resurfaces safely.
One of the most extreme kinds of diving is free diving. Free divers don't rely on a breathing apparatus at all. They simply have to hold their breath.
That's why GoPro footage of one free diver in a frozen lake in Canada is so amazing. Matthew Villegas needed to calculate exactly where he entered the water, otherwise he'd be trapped under ice.