Can someone ELI5 why the Hasidic guys dug the tunnel under Chabad, and why they got angry when the city was going to fill it in?

What was the motivation? Was it actually religious?

And what was the opposition to having it filled?

Was there any good reasons to have the tunnel built in the first place? If so why did they not just get proper permitting annd have it built professionally/to code initially?

Did it have anything to do with the mechitza/gender divide?

Can anyone eli5 what is going on or send me to a good article analyzing the whole thing?