I am willing to bet that the UN knew all about it.
Of course, nearly everyone who pays attention knows all about it. The UN in so many ways, through their aid agencies on the ground and flood of foreign cash, are complicit in it. They spent 15+ years building a network of tunnels and bunkers under civilian and humanitarian structures, to smuggle in and out of Israel and the Sinai, to use as command centers, and to store weapons caches.
Hamas doesn’t care about lives. Not their own, not their people’s. They are driven by murderous hateful religious ideology very similar to Al Queda and ISIS and the Ayatollahs. Everything and anyone is not just fair game in their religious war - more deaths, in their mind, is used to justify their propaganda and in theory rally even more support. They would love for Israel to bomb a hospital - which is why Israel probably won’t. Israel will have to go in with troops and sweep the hospital floor by floor and bunker by bunker. They should video it as they do; since the propaganda war is being fought alongside a real war.
Hamas needs to be eradicated. Ideally the Palestinians would rise up and get rid of them themselves. Israel maybe preparing the ground for them to do so. Or maybe they won’t.