Greater London is located in southeastern England, situated along the navigable River Thames, which bisects the city from west to east. The city was built in a geographical bowl known as the London Basin, flanked by the Chiltern Hills to the northwest and the North Downs to the south.
The geography of London is defined by its waterways. The Thames is the primary geographical feature, naturally meandering and historically dictating the city's growth, trade, and defense. Several smaller, often subterranean rivers—known as London's "lost rivers," such as the Fleet and the Tyburn—flow into the Thames.
Despite being a heavily urbanized megacity, London is remarkably green. Nearly 47% of Greater London is green space, encompassing Royal Parks, woodlands like Epping Forest on the northeastern fringes, and a vast protective "Green Belt" established in the 20th century to prevent urban sprawl. The underlying geology is dominated by London Clay, a dense, blue-gray soil that proved highly suitable for tunneling, a fact that facilitated the early development of the London Underground.