The Great Mystery of Fire Lake

Not too far from downtown Stuttgart, Germany, sits a very small park with a very small pond but a very huge cathedral:

What is NOT a mystery is what these things are: The pond is called the Feuersee (in German, “fire lake”) – and the cathedral is called the Johanneskirche.  The church was built in 1864, so it is relatively new – but you can still see some damage from bombing in World War II.  The pond was created somewhat earlier, in 1701. It was designed as a small reservoir for fighting fires, hence the name, Fire Lake.

What the MYSTERY is: how did this Fire Lake actually work?

In the middle of the pond you can see three intake pipes (one of them is shown in the photo above) – but where exactly did the water go?  How was it pumped out?  Was it transported to horse-drawn fire wagons, or was something more sophisticated in place?

Having living nearly 10 years in Germany, I’ve visited the Feuersee many times – but I’ve never seen any historical placques that describe how all this worked – or when/if they stopped using it.