Quantum mechanical tunneling of “heavy” atoms (i.e. heavier than hydrogen) is extremely atypical, but also exceptionally rich chemistry, especially at cryogenic conditions. The only requirements are a rather low activation energy and, much more important, a narrow barrier. Through the theoretical analysis of the quantum tunneling driven reactions we will evaluate the novel concepts of isotopic controlled selectivity, tunneling stability, inverse tunneling kinetic isotope effect, and some other cryptic effects.