This pool's surreal color isn't from magic, but from a unique combination of geothermal heat, dissolved limestone, and trillions of microscopic organisms.
The science of a surreal color
The color of Sra Morakot, or the Emerald Pool, comes from geology and physics interacting in a geothermal environment. The process begins deep underground where water is heated by geothermal activity. This hot water then moves upward through thick deposits of Permian limestone, a rock composed primarily of calcium carbonate (CaCO3). As the hot water flows through the rock, it dissolves the calcium carbonate and carries it in solution.
The water surfaces upstream at a source called the Blue Pool (Sa Nam Phut), where temperatures can reach 50°C (122°F). As this mineral-rich water flows downstream and cools on its way to the Emerald Pool, the change in temperature and pressure causes the dissolved calcium carbonate to precipitate out, forming a suspension of fine, microscopic calcite particles. These suspended particles cause the pool's color. They are the perfect size—typically between 0.1 and 10 micrometers—to scatter sunlight. This phenomenon, known as light scattering, affects shorter wavelengths (blue and green) more than longer wavelengths (red). The result is that the water reflects a turquoise and emerald hue back to the observer, an effect that changes with the angle of the sun. The water in the Emerald Pool itself maintains a pleasant temperature between 30°C and 40°C.
A unique geothermal ecosystem
The Emerald Pool exists within the Khao Pra-Bang Khram Wildlife Sanctuary, one of Thailand's last remaining lowland rainforests. This protected area provides a habitat for a diverse range of species, though it is most famous among ornithologists as the last known location in Thailand for the critically endangered Gurney's pitta (Pitta gurneyi). The rediscovery of the bird here in 1986 was a major event, but sadly, the species is now considered functionally extinct in the country.
The geothermal water creates a unique environment for microscopic life. Thermophilic (heat-loving) bacteria and algae thrive in the mineral-rich conditions. While the primary color of the pool comes from the limestone particles, these organisms form the basis of a unique aquatic ecosystem adapted to the warm, mineral-saturated water.
The water's journey begins at the Blue Pool, located about a 600-meter walk from the Emerald Pool. Swimming here is forbidden due to the high heat and a bottom of soft quicksand. A peculiar feature of the Blue Pool is that clapping loudly near its edge can cause a sudden release of bubbles from the spring below, a result of sound vibrations disturbing the carbon dioxide-rich water. This entire system, from the geothermal source to the mineral-laden pools, links deep earth geology and surface-level biology.
💡Fun Facts
The pool’s color intensity changes with the sunlight, appearing most bright on bright days.
The source spring, the Blue Pool, is so hot and its bottom so soft with quicksand that swimming is strictly prohibited.
Clapping your hands at the edge of the Blue Pool can cause bubbles to erupt from the bottom.
The surrounding wildlife sanctuary is one of the last known habitats of the Gurney's pitta, a rare bird now considered functionally extinct in Thailand.
The park is generally open daily from 8:30 AM to 5:30 PM, though specific pool access times may vary. It is best to arrive early to avoid crowds.
Admission
The entrance fee for foreign adults is typically 200-400 Baht, and 100-200 Baht for children. The fee includes access to both the Emerald and Blue Pools.
Accessibility
To reach the Emerald Pool, there are two main paths from the entrance: a direct 800-meter trail and a longer 1.4-kilometer scenic nature trail with a wooden walkway. The paths can be slippery when wet.
Sra Morakot's Invisible Life Hot SpringKrabi, Thailand