Geological and fossil evidence indicates that the Kathmandu Valley was covered by a large lake between approximately 2.8 million and 10,000 years ago. Paleo-Kathmandu Lake is thought to have drained in phases due to the drying of the regional climate; continuing mountain building-tectonic uplift and faulting-in the region; and the formation of an integrated drainage system, as river channels cut through previous rock ridge dams.
The green, vegetated slopes that ring the Kathmandu metro area (light gray, image center) include both forest reserves and national parks. The metropolitan area is relatively flat compared to the surrounding mountains. Tribhuvan International Airport, near the eastern margin of the city, is the only international airport in Nepal.
Archeological evidence suggests that the human development of Kathmandu, together with the nearby "sister cities" of Lalitpur and Bhaktapur, began almost 2,000 years ago. Today, the three cities form the governmental, cultural, and-as a main access point to the Himalayas for tourism-economic center of the country.