We took the 12:30 tour through the Grotten Noord. It was a weekend and there were around 15 people on the English speaking tour.
The tour guide seemed very knowledgeable and even cracked some jokes in Dutch humor :)
We walked for a bit over an hour and saw the subterranean cave system which was pretty impressive!
I do recommend to take a sweater/jacket with you. The walk takes about an hour and it can get pretty cold down there!
Nice experience though!
Afterwards one can lay in the park with some ice cream and warm up again in the sun!
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