Dutch inland lakes

Almost none of these lakes are natural. The Netherlands has been digging holes in itself for centuries and then letting them fill, and every generation of digging left a different kind of diving.

Peat. From the Golden Age onward, peat was cut for fuel and shipped to the cities. The Vinkeveense Plassen are the classic case: the turf went to Amsterdam, and what was left was shallow water divided by legakkers, the long narrow banks where the wet peat was laid out to dry.

Sand for the motorways. In the 1960s and 70s the same lakes were dug again, far deeper, for sand — the A2 motorway and the Amsterdam-Zuidoost development both came out of Vinkeveen, which is why a peat lake now has holes 50 m deep in it and a string of spoil islands, the Zandeilanden, along its southern edge. The same thing happened all over the country: Bussloo, Slijk-Ewijk, De Beldert, Bronsbergen and the rest are motorway and dyke-building borrow pits that were landscaped into recreation lakes afterwards.

River gravel. In Limburg the Maas floodplain was worked for gravel, leaving deep, clear-ish pits like the Boschmolenplas and De Grote Hegge at Thorn.

What they share: fresh water, a thermocline that in summer puts 20 °C over 6 °C, visibility that depends entirely on the algae and the last person to swim past, and an economy of sunken objects — boats, cars, aircraft, a bus, garden gnomes — put there by clubs to give people something to find. Most Dutch dive training happens in lakes like these.