London is divided into six travel zones. Zone one is the most central zone and zone six is the outer zone which includes Heathrow Airport. The more zones you cross, the more you pay on the Underground. To travel on the Underground, you can buy a daily ticket, a daily travel card, or an Oyster card. This is a smart card with an electronic chip that you charge with credit, and used to pay for travel on the Underground and on buses. It is the cheapest way of travelling in central London. Some Underground stations have lifts; most have escalators and stairs. The longest escalator in Europe is at Angel station on the Northern Line: it is 60 metres in length, with a vertical rise of 27.5 metres. People using the escalators stand on the right-hand side, so those in a hurry can walk past them on the left. There are several safety announcements given to passengers who travel on the Underground. When the doors of the trains are about to close, you hear ‘stand clear of the doors please’. When the train stops in a station where there is a gap between the train and the platform you will hear the famous phrase, ‘Mind the Gap!’