The hidden route goes over a mountain near Wang Prachan Border Market in the southern province’s Khuan Don district, which shares a border with Malaysia’s Perlis town, district chief Pheeraphat Ngerncharoen said on Friday.
Investigators found a wooden ladder hidden behind trees leading up the mountain for about 150 metres and after that, they found a short tunnel and a kilometre-long route leading into Malaysia.
Scattered empty water bottles and instant food packages show that the route was used recently, officials said.
A local villager, meanwhile, said that even though border police patrol the area regularly, they had no idea a secret route was hidden right under their noses.
Pheeraphat said he had been alerted by villagers who noticed cars and motorcycles stopping near the area several hours before the market opened. This had been happening for about a month, prompting villagers to notify authorities of possible illegal operations.
The district chief has ordered officials to tear down the ladder and for security personnel to be deployed on a 24-hour patrol. He has also notified the Internal Security Operations Command to find out if the route is connected to any known crime syndicates.