Yeah but if it was a cistern, why did whoever built it build what looked like a rather elaborate cooler space with a double bottom on top of it?
Also, while the level of the lake is higher today, it wasn’t that much lower back in the days. So if it was a cistern, it would have been very close to the waterline, and regularly flooded every year anyway like it is today: the level of the lake is regulated now, but it wasn’t before, and the level varied a lot more.
The location doesn’t make much sense for a cistern. Perhaps for a well, if whoever built it wanted clean lake water filtered by the sandy ground.


























That would indeed make sense. But what doesn’t is why somebody built this in what was uninhabited marshland at the time, and why they built a seemingly totally-unrelated cool storage box on top of it.