Sardinia's underground is a honeycomb of more than 300 caves and caverns, and this trip will take you to one of the most famous. Neptune's Caves were discovered by fishermen in the 18th century and named after the god of the sea. The caves can be accessed by road, but this tour will take you on the scenic route. You'll board a boat in Alghero and sail north along the coast. The journey takes less than an hour and sails you past the sheer cliffs at the Capo Caccia. When you arrive at the caves, you'll have the opportunity to purchase a ticket to go inside. A series of passages will take you through a cross section of the 4-kilometre cavern. You'll see stalactites biting down like incisors from the roof of the cave and stalagmites stabbing up from the floor like daggers. There's a salt water lake down here, too, which stretches out for 120 metres. After an hour or so of exploring, you'll return to the boat and sail back to Alghero.