A City With a Hollow Hill
South of Maastricht's centre rises the Sint Pietersberg, a hill of soft limestone known locally as mergel (marl). For centuries this stone was quarried underground, and by the 19th century the tunnels in the hill added up to roughly 200 km of passages. Surface quarrying in the 20th century destroyed part of the system, but large sections survive. Locals call them "grotten", caves, although they are man-made quarry galleries.
The city has a second underground network too: the casemates, defensive tunnels of brick and limestone built under Maastricht's fortifications. Both can only be visited on guided tours.
The Caves of the Sint Pietersberg
Quarrymen cut blocks for building stone by hand, leaving high, regular galleries with marks of their tools still visible. Over the centuries workers and visitors left drawings, inscriptions and signatures on the soft walls in charcoal and carving, and these are one of the highlights of a visit.
The caves also served as shelter in times of war. During the Second World War, part of the Dutch national art collection was hidden in the hill, including the paintings of the Mauritshuis, and Rembrandt's Night Watch was stored in a specially built vault until the liberation in 1945.
The tours run by Maastricht Underground currently include:
- Grotten Noord (North Caves), about 1 hour 10 minutes, available every day of the year
- Grotten Noord including the Kluis, about 1 hour 40 minutes, on selected dates
- Grotten Zonneberg, about 1 hour 10 minutes, on selected dates
- Zonneberg including the museum and the Night Watch story, about 1 hour 45 minutes
- A tour of caves used as a former NATO headquarters, about 1 hour 10 minutes
- A step tour going deeper into the Zonneberg system, about 1 hour 20 minutes
A children's tour of Grotten Noord for ages 4 to 10 runs from April to October.
Fort Sint Pieter
On the northern edge of the hill stands Fort Sint Pieter, built in 1702 after attackers had repeatedly used the hill as a base during sieges, including Louis XIV's siege of 1673. The fort has been restored to its original appearance. Tours of about one hour run every day of the year and take you through its galleries and onto the walls, which give a good view over the city and the Maas valley.
Combining the fort with the North Caves is the most common choice, as both are close together on the hill. The site also offers combination tickets.
The Casemates
The Kazematten Waldeck are part of a network of tunnels under the old fortifications, about 14 km in total. They were built so that defenders could move unseen, listen for enemy tunnelling and set countermines. Tours of about an hour run on selected dates and give a good sense of how Maastricht was defended as a frontier fortress.
What to Expect Underground
The tunnels are cool all year, so bring a warm layer even in summer, and wear sturdy shoes, as floors can be uneven and damp. The caves are dark and the guide leads the way with lamps; it is easy to see why the tours are guided only, since the older systems are a genuine labyrinth. Some tours involve steps and longer walks, so check the description if mobility is a concern.
Two tours in one day is a comfortable maximum. A cave tour followed by the fort fills a half-day, and walking back to the centre along the Jeker valley and through the Jekerkwartier makes a good end to the visit. Photography is usually possible, but the low light calls for a camera or phone that handles dark scenes well.
Planning Your Visit
Book online on the Maastricht Underground website, where regular tours can be reserved for groups of up to 14 people. Larger groups and private tours are booked separately. Several tours run only on selected dates, so check the calendar before you travel, and book ahead in school holidays.
The Sint Pietersberg is south of the centre and can be reached on foot in under half an hour from the Vrijthof via the Jekerkwartier, or by bicycle. The hill itself is worth time above ground: the former ENCI quarry, which closed in 2018 after almost a century of cement production, is being returned to nature, and the cliffs are home to breeding eagle owls.
