In America, it's called plumbers crack. In Britain, builder's bum.
In Japan, however, they're taking the guesswork out of exposing the butt-crack with panties that ride super low around the bottom of the butt. The company responsible for the low-riding undergarment is called Predator Rat, and they offer a variety of styles that might be sexy, but certainly don't look super comfortable.
Would you wear these?