Ask HN: How low is the ECB capable of pushing rates?

I've long wondered what would happen in a bad recession when rates are already super low, and now the possibility seems on the table that we will find out. The US has some wiggle room as it's rates aren't as low but the EU already has negative real rates and 0 funds rate. So lets say they saw a major sell off over the next couple weeks. Lets say upwards of 15%.(not suggesting this will happen, just that it's possible) They are going to have a strong incentive to make major rate cuts and I know they have explicitly said they are not opposed to this in principle. On the other hand, for example in the US, there are structural reasons that make it seem like it would be difficult to lower rates too or beyond zero. The biggest I can think of is that pensions rely on the ability to find meaningful yield to be able to meet their obligations. Ive heard of various other scenarios around banking stability and the repo market as well that are speculated could cause big issues . My subjective question to anyone with an opinion: does the ECB have any restrictions of this type, especially the risk of pension collapse or could actually test out a -2 or - 3 % percent rate without causing an obvious blow up. What weird stuff would happen at - 3%. Negative mortgages? Would the whole credit system start looking outside of the EU completely?