smog from factories and an ever-increasing fleet of automobiles, as well as a lack of adequate free space due to congestion, had evolved into the city's most important challenge. A series of anti-pollution measures taken by the city's authorities in the 1990s, combined with a substantial improvement of the city's infrastructure (including the Attiki Odos motorway, the expansion of the Athens Metro, and the new Athens International Airport), considerably alleviated pollution and transformed Athens into a much more functional city. In 2004, Athens hosted the 2004 Summer Olympics. Geography Athens sprawls across the central plain of Attica that is often referred to as the Athens Basin or the Attica Basin (). The basin is bounded by four large mountains: Mount Aigaleo to the west, Mount Parnitha to the north, Mount Pentelicus to the northeast and Mount Hymettus to the east. Beyond Mount Aegaleo lies the Thriasian plain, which forms an extension of the central plain to the west. The Saronic Gulf lies to the southwest. Mount Parnitha is the tallest of the four mountains (), and has been declared a national park. The Athens urban area spreads over from Agios Stefanos in the north to Varkiza in the south. The city is located in the north temperate zone, 38 degrees north of the equator. Athens is built around a number of hills. Lycabettus is one of the tallest hills of the city proper and provides a view of the entire Attica Basin. The meteorology of Athens is deemed to be one of the most complex in the world because its mountains cause a temperature inversion phenomenon which, along with the Greek government's difficulties controlling industrial pollution, was responsible for the air pollution problems the city has faced. This issue is not unique to Athens; for instance, Los Angeles and Mexico City also suffer from similar atmospheric inversion problems. The Cephissus river, the Ilisos and the Eridanos stream are the historical rivers of Athens. Environment By the late 1970s, the pollution of Athens had become so destructive that according to the then Greek Minister of Culture, Constantine Trypanis, "...the carved details on the five the caryatids of the Erechtheum had seriously degenerated, while the face of the horseman on the Parthenon's west side was all but obliterated." A series of measures taken by the authorities of the city throughout the 1990s resulted in the improvement of air quality; the appearance of