Renting an inflatable motorboat and cruising the Baunei Coast is a great way to enjoy the many pristine beaches, electric-blue waters, and snorkeling areas that make this corner of Sardinia such a summertime destination. Most boaters, however, zip right past Grotta del Fico.

This 800,000-year-old coastal cave system, which winds through a sheer limestone cliff looking out onto the Tyrrhenian Sea, is only distinguished by a wooden staircase protruding from its mouth, 30 feet above sea level. It’s a beacon to all speleophiles, the geologically inclined, or anyone looking to beat the heat, if only temporarily.

Given its long-unreachable location, Grotta del Fico went untouched by humans for millennia, and was known only by local fishermen until the 20th century. In 1957, a local shepherd descended the cliff to first penetrate the cave. Speleologists followed shortly thereafter, and continue working to this day, delving farther into the system’s many yet-unexplored tunnels. Geological splendor aside, researchers found that the cave—whose lowest reaches dip below the waterline—was once home to a small population of Mediterranean monk seals, one of the rarest marine mammals on earth (now critically endangered). A mother and pup still call the cave home, although they are taxidermied.

Opened to the public in 2003, the tunnel’s walking trails run 800 meters along the bed of an ancient river, granting visitors up-close views of a range of otherworldly stalactites, stalagmites, and delicate crystal-like sculptural formations. Tour guides are available for hire, though mobile devices are offered that guide visitors in a number of languages.

To access the cave, simply tether your boat to one of the dedicated anchor lines surrounding the base of the staircase, indicated by floating orange buoys. A boat will ferry you to the dock (free of charge), where you’ll climb the stairs and pay the fee (€10) before entering the cave system. If you’re lucky, you may be greeted by two resident cats. At a year-round 64 degrees Fahrenheit, you’ll likely need more than just a bathing suit.