The Hypogeum is an enor­mous sub­ter­ranean struc­ture exca­vat­ed c. 2500 B.C., using cyclo­pean rig­ging to lift huge blocks of coralline lime­stone. Per­haps orig­i­nal­ly a sanc­tu­ary, it became a necrop­o­lis in pre­his­toric times.