The lake is not old. Cheow Lan filled in 1987, when the Ratchaprapha Dam closed across the Phasaeng River and 165 square kilometres of valley floor went under. What survived above the waterline were the peaks. Those limestone karsts — some rising more than 900 metres from the surface — are roughly 300 million years old, remnants of a Permian coral reef that once sat beneath a shallow sea. The reservoir simply rewrote their base. Villages, forest tracks and elephant corridors lie beneath the jade-green water; the drowned trees still surface in the dry season as pale spars. A cheow lan lake tour from krabi therefore crosses a landscape with two different clocks running at once. Khao Sok National Park, gazetted in 1980, protects around 739 square kilometres of the oldest evergreen rainforest on earth — a forest system frequently dated at 160 million years, older by a wide margin than the Amazon. It escaped the Pleistocene ice ages that thinned equivalent forests elsewhere. Botanists come for Rafflesia kerrii, the parasitic bloom that opens to nearly 80 centimetres and appears without stem or leaf, and for the dipterocarp canopy that closes above the trail systems. Why the place matters now is partly a question of scale. Khao Sok connects to Klong Saeng, Khlong Nakha and several adjoining reserves, forming one of the largest contiguous protected blocks in southern Thailand — habitat for Malayan tapir, gaur, clouded leopard and hornbills that cross the water in noisy pairs. The lake is the access route into that block. Longtail boats, powered by exposed truck engines and long propeller shafts, remain the only practical transport; roads end at the dam wall. Access carries a 300 THB national park conservation fee per foreign adult, and the park operates 06:00–18:00 daily. The built layer is modest and deliberate. Floating raft houses, tethered in sheltered inlets, sit low on bamboo pontoons and rise and fall with the reservoir's seasonal drawdown. Guides steer visitors toward Khao Sam Kloe, the three-pronged formation locals compare to Guilin, and toward Nam Talu and Diamond Cave, where stalactite chambers hold constant temperature year-round. Khao Sok lake day trips and Krabi day trips alike converge on the same pier at Khao Phang, in Ban Ta Khun District. The park's own literature is blunt about the trade-off the dam represented: hydroelectric capacity purchased at the cost of a valley. Visitors on Khao Sok National Park tours are looking at both the loss and what the loss preserved — a flooded basin no road can reach, and karst walls no logging concession ever touched. Demand for a cheow lan lake tour from krabi has grown steadily against that backdrop, and the boats that carry it now double as the park's most reliable patrol network.