CO2 tables build genuine breath hold tolerance over weeks. Hyperventilating beforehand does the opposite of what beginners think it does, and it has killed trained divers.
Frenzel and mouthfill solve the one problem that stops more beginner freedivers than fitness or fear ever does: equalising the ears under rising pressure.
When a diver surfaces with decompression sickness, a hyperbaric chamber is the only real fix. Here is how recompression works, and where chambers actually are.
