Title: London
Content: a major international air transport hub with the busiest city airspace in the world. Eight airports use the word "London" in their name, but most traffic passes through six of these. Additionally, various other airports also serve London, catering primarily to general aviation flights. The London Underground, commonly referred to as the Tube, is the oldest and third longest metro system in the world. The system serves 270 stations and was formed from several private companies, including the world's first underground electric line, the City and South London Railway. It dates from 1863. Over four million journeys are made every

Title: Transport in London
Content: route around the city and incorporates the oldest part of the Underground's history, the Thames Tunnel under the River Thames, which was completed in 1843. The routes comprise: Other routes operated include: Heathrow, Gatwick and Stansted airports are served by dedicated train services, and are also served by standard commuter services. The Heathrow Express service from Paddington is provided by the airport operator, Heathrow Airport Holdings, whilst the Gatwick Express from Victoria and Stansted Express from Liverpool Street are provided by franchised train operating companies. Unlike the Underground, which is a single system owned and operated by Transport for London,

Title: Economy of London
Content: project is construction of 42 km (26 mi) of new tunnels connecting stations in central London including a branch to Canary Wharf in east London. It is Europe's biggest construction project with a £15 billion projected cost. An additional line, Crossrail 2, has been proposed. London is served by six international airports which are the world's busiest city airport system by passenger traffic. These are Heathrow, Gatwick, Stansted, Luton, London City, and Southend. There are a number of proposals for expanding airport capacity for London including expansion of London Heathrow Airport and expansion of Gatwick Airport. The principal argument in

Title: Airports of London
Content: cannot be expanded due to the docks on either side. It is also the only airport serving London which does not operate at night. A light rail service from London City Airport DLR station, which adjoins the terminal building of London City Airport, links it to the financial district of the City of London at Bank and Monument stations, which offer interchanges with London Underground. Located in the London Borough of Hillingdon, Heathrow is by far the largest of London's airports and considered the main gateway into the United Kingdom for non-European visitors. Heathrow has four terminals and two parallel

Title: Transport in London
Content: city in the London Borough of Hillingdon, it has two runways and five passenger terminals, with the £4bn fifth terminal opening in 2008. It is connected to central London by the dedicated Heathrow Express rail service, the Heathrow Connect local rail service and London Underground's Piccadilly line, and is connected to the M4 and M25 motorways. Gatwick is just under 40 km south of central London in Sussex, some distance outside London's boundary. With a single runway and two terminals, it handles approximately 32 million passengers per year from domestic, short-haul and long-haul flights, and is linked to London by

Question: Which of London's four airports is the only one to be connected to the city Underground system?
Answer: Heathrow