Which Italian sea cave is famous for its blue light?

Which Italian sea cave is famous for its blue light? The Blue Grotto, or Grotta Azzurra, on the island of Capri. Its intense blue glow is created by sunlight entering through an underwater opening and passing through the surrounding seawater.

The cave has two important openings: a small above-water entrance and a larger submerged opening beneath it. Because the underwater opening sits below the surface, it admits filtered sunlight that illuminates the water from below. Water absorbs more of the red wavelengths, leaving the reflected light predominantly blue.

The entrance is extremely narrow—about two meters wide and roughly one meter high at low tide—so visitors traditionally enter lying flat in small rowing boats. Access depends on calm seas and low tides. The cave extends around 50 meters into the cliff, while published measurements describe the interior as approximately 60 meters long.

Capri’s Blue Grotto is not the only cave called a Blue Grotto; similarly named sea caverns exist in Malta and elsewhere. The question’s reference to the famous Italian cave points specifically to Capri’s Grotta Azzurra. The surrounding island also gave its name to the color “Capri blue.”