Grotta del Bue Marino is a cave located in the Gulf of Orosei, on the eastern coast of Sardinia, Italy. The cave is known for its impressive size, measuring about 1,800 meters in length and 70 meters in height. It was discovered in 1957 and opened to the public in 1961.

The cave takes its name from the monk seals that used to inhabit the area and are known locally as "bue marino," which means "sea ox." The cave is home to an underground river, which has created a series of large chambers and tunnels with stalactites and stalagmites.

Visitors can take a boat tour through the cave, which lasts about an hour and covers a distance of around 1.5 km. The boat ride takes visitors through a series of chambers, including the Sala del Canto, where the acoustics are particularly impressive, and the Sala delle Frecce, which features a large stalagmite in the shape of an arrow.

Grotta del Bue Marino is a popular tourist destination in Sardinia and is considered one of the most beautiful caves in Italy.