Which London Underground line is the only one that runs entirely below ground?

The Waterloo & City line is the only London Underground line that runs entirely below ground.

The short line links Waterloo station with Bank in the City of London. It opened in 1898 as the Waterloo and City Railway, giving commuters from the main-line station at Waterloo a direct connection to the financial district. The line has only two stations and no intermediate stops.

Unlike most other Underground routes, the Waterloo & City has no section in the open air. Its tunnels run beneath central London for the whole journey. The line is sometimes called “the Drain,” a nickname connected with its short underground route and its historic role as a commuter link.

The Waterloo & City is also unusual because it does not operate on Sundays or public holidays under normal service patterns. It is primarily designed for weekday commuting, although service arrangements can change during engineering works or special events.