You're not kidding. We're several weeks out from our trip, and I thought I'd poke around and see of what you spoke. I was dismayed to find the Hypogeum sold out into the 2nd week of May with several subsequent dates already sold out.

"The Hypogeum of Ħal Saflieni is a very popular tourist attraction. However, because of its age, Heritage Malta (the government body that looks after historical sites) only allows 60 persons per day to visit the Hypogeum. Heritage Malta recommends tourists to book well ahead of time if they wish to visit. "