When we boarded the Oh-Yeah boat in Mellieha (for the Gozo trip, including Blue Lagoon and Crystal Lagoon), there was a salesman on board asking each passenger if they would like to buy a 10E speedboat ticket that would take you into the caves and tunnels near the Blue Lagoon in Comino. We bought two tickets, not wanting to miss any valuable experience. The 20 minutes in the speedboat does eat into your swimming time in the Blue Lagoon, but it was well worth it. There are several caves/tunnels to explore, and the dreadlocked driver made the trip more exciting by deliberately swinging the speedboat from left to right! (We were warned to protect our cameras from the splash.) Despite being senior citizens, we considered this addition to the trip well worth 10 Euros.