Mystery Underground - J. Jefferson Farjeon

The King's Cross restaurant is nearly empty when a timid London clerk overhears a few muttered words at a neighbouring table - a young woman is to be followed, and dealt with roughly if she proves troublesome. Before he can talk himself out of it, he is on a northbound express, pursuing the men and the girl they mean to harm.

The journey ends on the windswept moors of Northumberland, where a tumbledown house sits alone against the sky. An out-of-work navvy has already taken shelter there, and soon a pair of young adventurers join the vigil. But the house is not as empty as it appears. Something beneath it gives a hollow boom, a professor works against his will, and a ruthless schemer will stop at nothing to protect what is buried below.

First published in 1928, this thriller of treason, courage, and a secret that must never see daylight is vintage British mystery at its most atmospheric.