Malta is made up of lots of limestone — the skeletal remains of algae, corals, and mollusks that fell to the bottom of the sea floor over time. The lumpy formation at Wied iż-Żurrieq (the launch point for Blue Grotto excursions) is known as the Lower Coralline Limestone layer and was laid down 35 million years ago. It’s pitted and tunneled almost like a termite mound, a result of weathering by saltwater and acid rain over the years.