Valkenburg, in the southwest of the Netherlands, boasts a unique 10km underground cycle path. The entrance to the mine-shafts and tunnels is reached by Holland's only ski chairlift. It is exited by a toboggan run.
Europe's only public subterranean bike trail happens to be directly below Holland's highest point, Wilhelmina Tower, which overlooks the province of Limburg and commands views of the other Low Countries.
There were 10 of us on the expedition to the earth's core.
Freek rode post. Ger, the other charming guide, cycled up behind me to shout: "The Romans mined the caves for limestone or marl. Most of the local buildings are built from it. These caves were once used by the Army for survival training. Now it's a one-of-its-kind activity centre. The space hasn't been wasted."
We pedalled into the blackness, descending into the bowels of Valkenburg. At their widest, the tunnels are still less than 2m. At their narrowest, they are less than 1m. Maximum headroom is about 1.5m. You quickly develop a widow's hump cycling beneath the Netherlands.
"You have to get off and push sometimes as the roofs can be low and the turnings quite tight," said Freek as we rounded another bend to be confronted with more pitch dark. I could just make out the occasional beam overhead and other architecturally uninspiring roof supports.
Apart from that, it was just a stretch of damp wall-face. And the Rubensesque buttocks of the Belgian lady in front.
Occasionally we passed some graffiti. We didn't pass any skeletons. There were no decomposed bodies with bicycle clips surrounded by energy bar wrappers. The trip went well. It was uneventful. No one starved to death. No one threw a claustrophobic wobbly.
"Your eyes soon get used to the lack of light and the sights can get pretty spooky," added the engaging Freek as we bumped under more low ceilings.
Guided walking tours through the caves are also available, although people with respiratory or heart problems - as well as abnormally high levels of common sense - are advised not to try to journey down to the not-quite centre of the earth.
An escorted ride on the Dutch underground cycle path takes around 90 minutes, depending on punctures and how many times you black out when you hit your head on the ceiling. The tunnels also have a 50sq m climbing face, two vertical abseiling routes, a rope bridge and a "toggelsite" cable ride.
Twenty minutes from Maastricht, Valkenburg is a popular holiday resort. The Gemeentegrot cave sheltered 15,000 people during World War II. There are also catacombs, the Therma 2000 spa and the Drielandenlabyrinth - The Maze of the Three Counties - Europe's biggest maze.
Taking a breather, Freek talked up the indoor all-weather subterranean complex. His business is all about bums on seats. "The temperature is a constant 14C and it never rains. You can cycle to your heart's content every day of the year. You'll never ever see a cloud in the sky."
We made it back to the surface. Safely.
No one was lost. No one went the wrong way and got incinerated by magma. Or was attacked by giant insects or prehistoric monsters. No lost world was discovered. Nor any proto-human civilisation encountered. Vast underground seas and enormous mushrooms were conspicuous by their absence.
No alarm bells had sounded. No one had had to tinkle.
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Further information: To find out more about cycling underground in Valkenburg, see aspadventure.nl.
