Would it be a water-damage/mold-risk to leak 100l later in there? Or will it just soak away through the ground?
I'm considering adding an ice bath in a large barrel eg. 100l water. Incase it leaks in my natur-keller will the water naturally leave through the ground or how does a nature-keller work in Swiss houses. I have a house from your 2000 that has a natur-keller. It's usually 16-18deg with high humidity because of open ground (no concrete floor, just loose small stones).
You'll be fine. 100L is nothing.
Last summer during a hail/rain storm, the drains couldn't cope and our cellar was flooded with water around 20cm deep.
Apart from the mess and damage to the bottom of some furniture and paintwork, there was no other damage.
I'd put a lid on the bath though and buy a humidity meter as you are more likely to get problems with mould on walls and ceilings from constant high humidity.
If you plan on spending time in it make sure the house or this basement has had a proper radon measurement in the past. If you are in the wrong place it's perfect for getting lung cancer.
If the water leaks you can run dehumidifiers 24/7 for a while. Mold should not get worse if you dry everything properly and timely.