It's no secret that a massive supply glut has caused global oil prices to crash this year. Ferocious production from OPEC and near-record U.S. output is adding to sky-high oil inventories around the world.

But what's less widely known is that the oversupply problem has gotten so bad that oil tankers waiting to be offloaded are piling up off the U.S. Gulf Coast because there's nowhere to put the crude.