Five mainland women were found inside a secret compartment under the trailer of a container truck at a border checkpoint yesterday. They are suspected to have been destined for the sex trade in Hong Kong, according to Chief Inspector Chow Chun-fung, Ta Kwu Ling assistant divisional commander (operations). The five women, aged 18 to 25, were packed inside a metal box under the trailer when the truck arrived at the Man Kam To border checkpoint from Shenzhen on Sunday night. 'The compartment is five feet long, two feet wide and 2.5 feet high. They were crouching or sitting inside. One side of the metal box was open for them to breathe,' Mr Chow said. 'The box is believed to have been newly installed. It is the first time we have detected such a purpose-built and sophisticated structure for human smuggling under a goods vehicle.' Police arrested a 37-year-old Hong Kong driver. Last night he and the five women were being held for questioning at the border police station. The force's Under Vehicle Surveillance System operates at three border checkpoints - Man Kam To, Lok Ma Chau and Shataukok - to combat the problem. Mr Chow said every vehicle has to undergo an inspection when it arrives in Hong Kong from the mainland. On Sunday, two officers became suspicious when they discovered the illegal structure under the trailer and ordered the driver to stop for inspection. More than 3,000 Hong Kong-bound vehicles a day arrive via the Man Kam To checkpoint. In the first quarter of this year, police arrested 1,810 mainland illegal immigrants. There were 8,322 mainland illegal immigrants detected last year and 8,476 in 2000.