Yesterday in Maastricht I went to the `Mergelgrotten´, a massive series of man-made caves. The stone was discovered to be a good building material by the Romans and it has been quarried as a building material for the city ever since. The corridors have been used as shelters and even have 'bedrooms' and a fire place. On the Belgian side of the caves, the beer brewer Celis matures his Grottenbier in them and he is nog the only person taking advantage of the stable (11 celsius) temperature in the caves. Local farmers use certain parts of the caves to grow their vegtables on the tour we passed these mushrooms being formed. Apparently it only takes two weeks for them to mature properly in the caves.