Dark at the End of the Tunnel

Overlooking Maastricht from the south, the raised tableland of Sint Pietersberg is a great area for hiking, guarded by a hefty brick fortress. But even more intriguing is the phenomenal labyrinth of quarry-tunnels beneath, an amazing feat of mostly pre-industrial engineering. Marlstone and limestone have been laboriously hand-cut here since Roman times, and at one stage, there were 20,000 separate underground passages stretching past the Belgian border, with a total length of some 230km. They offered refuge in times of war, a smuggling route in pre-EU days, and a section was long used as a secret NATO command base. Over a third of the maze remains intact and there are several ways to visit, always by guided tour: getting lost here would likely be fatal. Walking through the tunnels is an eerie experience and you'll feel a deep chill if you're not suitably dressed; it's 11°C year-round with 95% humidity.

The most frequent tours visit the North Caves, where guides focus particularly on the tunnels' role in Napoleonic French history. Although not always accessible, ask the guide and you just might be able to visit the extraordinary vault where 750 paintings were stored for safe-keeping by Nazi occupiers during WWII. These included Rembrandt's The Night Watch, kept rolled in a tapestry and transported here in a coffin. Check tour times through Maastricht Underground and consider prebooking at busy periods. North tour departures are from the Fort St Pieter ticket booth. Use the same website for details of less frequent tours to the taller, equally intriguing Zonneberg Caves decorated with generations of charcoal drawings, from ancient Roman stick figures to wartime depictions of movie stars. Visits start behind the Buitengoed Slavante cafe some 3km south of town (free parking).