Freediving

Training tolerance for low levels of O2 is a very slow and long process apparently. It takes a lot of training and dedication. But I guess as most sport performance do. "Fun" fact though, the human body dosen't send any warnings for low level of 02, it just suddenly shuts down (black out). High level of C02 (High partial pressure technically) is what makes us feel the need to breath. Tolerance to C02 is way easier to train.