The Greek islands are already well known for their deep blue, clear waters, white sands, and hidden coves. If you’re looking for a deeper exploration of the waters surrounding the Greek islands, you might want to seek out a hidden swimming spot in one of Greece’s blue caves. While there are many caves that swimmers can explore, the island of Kastellorizo has one the most secluded blue caves in Greece, known as the Kastellorizo blue cave.

The Kastellorizo blue cave has two extraordinary features: First, the entrance is so small that it’s barely visible from the outside. Secondly, the way that the sunlight that can reach the inside of the cave reflects off the water turns the stone walls a deep, almost otherworldly shade of blue. In Greek, the cave is known as phokialí, or seal’s refuge.

Getting inside the cave isn’t easy, so you need to be a determined adventurer to make it work – you also might not want to attempt it if you’re claustrophobic. The entrance is about three feet high, meaning most adults will have to lay flat in the boat or hunch over completely just to make their way inside the grotto. Because the entrance is so small, the seas need to be calm before you can make the trip.

Once you get inside though, the views are worth it: The translucent dark blue of water and cave walls is astonishing to behold, and the high ceiling of the cave makes it resemble a cathedral. That’s why the best time to visit the cave is early in the morning, when the sun is still low.

Visitors can head to Megisti harbor to book a private tour that will take them to the cave. A speed boat will take your group to the cave – just be sure to confirm ahead that you’ll be allowed to hop off the boat and go for a quick swim. Some tours will also drop groups off at the small island of St. George, near Kastellorizo, where they can enjoy a leisurely day swimming before getting picked up again.

What to do in Kastellorizo, Greece

Besides visiting the blue cave, the peaceful island of Kastellorizo is worth taking the time to explore. Like so many other small towns in Europe and the Mediterranean, the cobblestone streets are full of charming detours.

Wander among the pastel blue and yellow mansions set on the hillside, or visit one of the five churches on the island. The alleyways in Kastellorizo are sometimes decorated with vines of pink flowers. Head to the island’s old town neighborhood to the 14th-century Castello Rosso. In fact, if you’re interested in learning more about ancient Greek history, there’s an archeological museum on the island, too.

This is a Mediterranean island after all, so if you want to visit Kastellorizo, plan a day of lounging on the beach, and eating fresh fish, prawns, and octopus.