Complex sentence: The highest point above ground on the Underground is the Dollis Brook Viaduct over Dollis Road between Finchley Central and Mill Hill East on the Northern Line, 18 metres (60 feet) above the ground.
Split into simpler sentences: The highest point above ground on the Underground is the Dollis Brook Viaduct over Dollis Road between Finchley Central and Mill Hill East on the Northern Line. It is 18 metres (60 feet) above the ground.