Some of you may already be aware of Wim Hof - he's a Dutch guy with world records for sitting in ice tubs, running marathons in the desert without water and swimming under ice.
But for me, the most impressive thing he did was to take a group of 20 normal people hiking up Kilimanjaro (5850 metres) without proper standard preparation, or wearing proper Winter clothing, and taught them how to survive that completely well.
The training he recommends is frequent cold water exposure, coupled with a breathing technique where you alternately hyperventilate and then hold your breath.
You can find out more about Wim Hof extremely easily with a Google Search - that's not what I'm sharing here
Many people have experienced good effects from the breathing technique, but it has been surprisingly unclear what actually is happening with the body, and actually Wim Hof himself has been very vague (or contradictory) when he discusses this. Many people incorrectly think for example that it "charges you with Oxygen".
However, some scientists have done experiments, and other people have been putting the pieces together to explain which parts actually work, and why.
Here are two resources - the first that I authored and a second that I found more recently that I also like, that are a great starting point for anyone interested in Science of the Wim Hof Method breathing technique: