A cenote (or sink hole) results from the collapse of limestone that exposes ground water underneath. This is from the effect of a meteorite 65 million years ago.
The water is usually very clear and comes from rain water filtering through the ground.
This one had many cat fish swimming about in it and as we didn’t realise we would be coming across one at the Mayan village we visited I ended up having to jump in in my underwear. So refreshing and totally worth it.