One has to pre-book a visit to this site, as only 80 people are allowed inside daily. Also, photographing is forbidden, so I can't show more here than its entrance and the ticket.
Is it worth all the fuss beforehand? Yes - besides being one of the highlights of Malta (together with Mdina, which still only is on the Tentative List), it's a unique construction dating from a very early age. Especially the "doors" are carefully carved out of stone, by people that didn't have metal tools in use.
There are a lot of mysteries surrounding the Hypogeum, adding some extra flavour to a visit. A trip here starts with a video that links this underground site with the many megalithic temples above ground elsewhere in Malta.