The Lich's Well of Power is located in an abandoned subway. The Lich uses the well to regain his evil powers after being imprisoned in amber. It is similar to the concoction of gasoline, bleach, plutonium, and other hazordous chemicals that The Lich uses in the episode "Mortal Folly." This well could have been left over from The Great Mushroom War. It can poison anybody, like what happened to Princess Bubblegum.
It appears to be set in an area that once turnabout for a subway trains due nearby. The well is fed by a dripping pipe above it, and, according to Jake, it is filled with "toxic waste."