Shocking photos of a popular beach in the Mexican state of Oaxaca sinking into the Ocean have been doing the rounds on social media, leaving people scratching their heads in disbelief. It turns out the sinking is being caused by a phenomenon known as “mar de fondo”, or “swell”.

Swells rarely occur at popular seaside destination, which only makes the repeated sinking of the beach at Puerto Escondito, in Oaxaca, that much more peculiar. Swells, also known as surface gravity waves, are created by storms thousands of miles away from the beaches on which they break, and are not influenced by local winds. They cause a rise in sea level, but are different than high tide, eating away at the sand they it, and causing large stretches of beach to simply sink into the ocean. This is what happened at Puerto Escondito last week, but the really strange part is that the exact same thing occurred in the same place, in May of last year.

Swells occur all over the world, but they are particularly strong on the coast of Mexico, as the strong winds that blow between Australia and the American continent push the waves to travel distances of more than 10,000 kilometers, without any obstacle, until they reach the beaches of Oaxaca and Guerrero. However, with the exception of Puerto Escondito, they do not affect popular seaside destination.

Infobae reports that swells can seem deceivingly harmless, as waves are not particularly high, so they are indistinguishable from regular ones,  due to their considerable wavelength, they produce an increase in sea level and cause serious erosion of sand strips, as you can see in these photos.