An iPhone-thief shut down two subway lines for an hour when he led cops on a wild chase down the tracks under Atlantic Avenue, authorities said.

The drama unfolded at around 6:40 p.m., when a 19-year-old snatched the gadget from a woman waiting for a southbound R train at the Pacific Street station in Brooklyn — and the gutsy victim gave chase.

Cops ordered the power shut down and a police dog helped them track the thief to his hideout, between the Pacific and Union street stations, sources said.