Compounds that make a cork waterproof?

Good Question!

In olden days, corks were lubricated with animal fat to make them easier to slide into a wine bottle. That is where the practice of "sniffing the cork" when accepting a bottle of wine comes from. If the cork smells rancid (rotted meat?) then you can be sure that air has gotten past the cork and probably to the wine, turning it as well.

When you are offered the cork next time at your favorite restaurant, offer that story and see if the sommelier comes to smile your way!