But when Gregory threw open a pair of doors and let in a red subterranean light, Syme was still smoking with one leg thrown over the other, and had not turned a yellow hair.
Gregory led him down a low, vaulted passage, at the end of which was the red light.

It was an enormous crimson lantern, nearly as big as a fireplace, fixed over a small but heavy iron door.

In the door there was a sort of hatchway or grating, and on this Gregory struck five times.

A heavy voice with a foreign accent asked him who he was.

To this he gave the more or less unexpected reply, "Mr.Joseph Chamberlain." The heavy hinges began to move; it was obviously some kind of password.
Inside the doorway the passage gleamed as if it were lined with a network of steel.