The pyramids of Mexico's Teotihuacan have always been one of North America's archaeological treasures. But those and other remains of the ancient city have always been a mystery too. A door into Teotihuacan's shrouded past cracked open a little this year when an archaeological bot equipped with a camera was sent on a subterranean trek and found a 12-ft. wide corridor with a perfectly preserved arched roof that was built — and sealed — nearly two millennia ago. Archeologists are hopeful that it may be connected to the tomb of a high priest, a find that could reveal volumes about how the people who constructed the Mesoamerican metropolis once lived.