These are hunebedden - either "Hun beds" or "giants' beds" (it's not entirely clear which the word derives from, nor is it clear that there was a distinction in popular mythology):
Fifty+ of them are found in the woodsy province of Drenthe in the north of the country, and they are in fact Neolithic burial mounds, belonging to the Funnel Beaker People and built around 5000 years ago. The rocks themselves are erratics weighing up to 25,000 kilos, swept down from Scandinavia by glaciers during an ice-age 200,000 years ago.
This is the biggest hunebed: D27
It was excavated in 1685 by a LGBT poet called Titia Bronsgerma, who was famous in her time for writing poems in alternating lines of French and Frisian.
Here's a rather wonderful engraving of her supervising the dig and being presented with treasures, dressed as a Greek goddess. Clearly archaeology was a lot more about having fun, and a lot less about post-holes and carbon-dating, back in the early days.
She of course subsequently wrote a poem ("Loff op 't Hunnebed") about her site, which is, ahem, loosely translated here.
This is what they are assumed to have looked like from inside when complete. Each would have had multiple occupants:
The Dutch are, of course, still obsessed with civil engineering and moving rocks about. They even build statues to rock-humpers:
|But they seriously need to learn to lift with the knees|