What are the most dangerous things you could jump into? Why is liquid mercury so dangerous?

How can lake Natron located in Ol Doinyo Lengai turn you to stone?

Why is potassium and sodium carbonate such a dangerous mix? What can stingrays do to you if you jumped into a pool of them?

How scary is a pool of sharks? Can you even survive a fall into a volcano? Would you cool off in a pool of liquid nitrogen?