In Siegburg nearby Cologne there is an indoor Dive Center called Dive 4 Life. I payed 50 Eur and could dive the whole day – obviously I did not go by myself but with my dive center Seekuh. 🙂
It’s been a funny experience, having tens of divers in one deep tub blowing bubbles! If you were on top of a few divers the bubbles could really interrupt your buoyancy haha! The tower has been decorated with old greek architecture and at about 6 meters a shark was swimming (no real one!). One can go down to 20 meters which I have not been in a long time so that was quite something 🙂 I also tried to take underwater photos for the very first time and it took me a while to get the hang of it.
If you are not the kind of diver to go diving in lakes – this is a great alternative to not going diving in between dive trips / holidays!