This stunning 165-sq-km lake sits 65km (an hour’s drive) east of park headquarters. It was created in 1982 by an enormous shale-clay dam called Ratchaprapha (Kheuan Ratchaprapha or Chiaw Lan). Limestone outcrops protruding from the lake reach up to 960m, over three times higher than Phang-Nga's formations. Most lake visits involve a day or overnight tour (including transfers, boats and guides).
Charter boats (2000B per day) from local fisherfolk at the dam's entrance to explore the coves, canals, caves and cul-de-sacs along the lakeshore.
Two caves can be accessed by foot from the southwestern shore. Tham Nam Thalu contains striking limestone formations and subterranean streams. Visiting during the rainy season isn't recommended; there have been fatalities. Tham Si Ru features four converging passageways used as a hideout by communist insurgents between 1975 and 1982.