The Kathmandu Valley You're Walking Through Used to Be a 200-Metre-Deep Lake

Most visitors landing at Tribhuvan International Airport look down at a bowl of terracotta rooftops ringed by green hills and assume it has always looked roughly like this. It hasn't. For the better part of a million years, the ground under Thamel, Durbar Square and the runway itself sat at the bottom of a lake.

Geologists call it the Paleo-Kathmandu Lake, and the physical evidence for it sits under your feet on almost any walk through the old city: thick layers of fine-grained clay and silt, the kind that only accumulates on a lakebed over tens of thousands of quiet years. Boreholes drilled across the valley floor turn up the same sediment sequence almost everywhere, and it only thins out once you climb toward the hills that ring the basin — Swayambhunath among them, which the Swayambhu Purana describes as the hill formed where the lake's central lotus, and the self-arisen flame it carried, came to rest as the water fell.

The draining wasn't a single dramatic event. As the last Ice Age cooled and dried the Himalaya starting around 30,000 years ago, the lake began to shrink, and over the following millennia its outflow cut steadily deeper through limestone at Chobhar — the valley's one natural drainage point. The lake didn't vanish all at once either: it shrank in stages, leaving three smaller lakes behind at Gokarna, Pashupati and Kitni before the valley floor finally dried into the flat, fertile plain that made Kathmandu, Patan and Bhaktapur possible in the first place.

What this means for trekkers

You can trace the old shoreline logic in a single day: the Kathmandu Valley Cultural tour climbs from the old lakebed up to Swayambhunath's hilltop stupa, tracing close to the line the water once stood at, before longer itineraries head onward into the mountains on a full Kathmandu and Pokhara tour.

Sources: Nepali Times, 'The lake that was once Kathmandu'; Swayambhu Purana tradition as summarised by Nepali cultural heritage guides on the Manjushree legend.

Cover photo: Bijay Chaurasia via Wikimedia Commons (CC BY-SA 4.0).