The Liège stone quarries

The province of Liège was quarried for petit granit — the blue-grey Carboniferous limestone that faces half the buildings in Wallonia — and when the working faces went below the water table the pits filled. La Gombe at Esneux and the Carrière de Lillé at Sprimont are two of them, run as dive centres with platforms, lines and sunken objects at known depths.

They are cold, they are deep for their size, and they are the reason a lot of Dutch and Belgian divers get their deep and technical training done without leaving the country.