A supertide has turned France's famed Mont Saint-Michel into an island and then retreated out of sight, delighting thousands of visitors who came to see the rare phenomenon.

The so-called "tide of the century" actually happens every 18 years. Although the tide rushes in and out along the whole northern French coast, it's especially dramatic at the UNESCO world heritage site, which is normally linked to the mainland only by a narrow causeway at high tide.

The high tide, said to rise at the pace of a horse's gallop, turned the Mont briefly into an island Saturday, while the day's low tide allowed people to walk on the expansive flat seabed.

Tidal specialist Nicolas Pouvreau told France 24 the surge was a few centimeters (inches) short of expectations.