Khao Nam Khang Historic Tunnel sits in the forested hills east of Hat Yai, near the Malaysian border, where communist insurgents once dug a concealed base into the clay. Tourism Thailand describes it as the largest and longest tunnel in Thailand, and the site now works as a modest historical park rather than an active military relic. Visitors walk through low passageways, former rooms, and interpretive displays that explain how the hideout functioned during the Cold War-era conflict in southern Thailand.
The attraction is not polished, but that is part of its value: it gives Hat Yai a historical counterpoint to markets and city food streets, and the drive into Khao Nam Khang National Park adds a sense of the terrain that made the tunnels useful. Go with your own transport or a driver, and treat it as a half-day excursion rather than a quick city stop.