A predawn fire yesterday wiped out the Siam Society's office and shop.
Parts of the society's valuable library were also destroyed.
The damage is inestimable given the fact the library had rare books, historical documents and precious records, especially those related to anthropology, archaeology, sociology and the environment.
Fortunately, there were no casualties.
"I heard crackling and a spitting sound at around 4am," said security guard Jaroon Singthep.
"I was about 10 metres away from the building."
He soon noticed a spark of fire before a huge plume of fire burst out.
"I tried to extinguish the fire with water, but that did not work, so I rushed to alert my supervisor," he said.
Police were notified at around 4.30am, after which more than 10 fire trucks rushed to the scene.
It took 20 minutes to put out the fire, which engulfed the whole building and caused some parts of the roof to collapse.
Pol Colonel Somprasong Yentuam, superintendent of Lumpini police station, said it would take 30 days to determine the cause of the fire.
"The Scientific Crime Detection Division is working
on evidence," he said.
On its website, the Siam Society said staff were assessing the damage and making recovery plans.