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A previous ANU study found waves off the town of Mundaka in northern Spain vanished because of changes to a sand bar after dredging in a nearby river.

A few sand bars had become visible, and dry rocks showed that had previously been under water.

At one point the overloaded boat appeared to be beached on a sand bar by the retreating tide but then it moved further out to sea.

Asphalt could be seen on every river island visited, ranging from globs stuck on riverside vegetation to thick mats of tar oozing across sand bars as summer temperatures heated it into a viscous liquid.

"I think they were just swirling on top of the sand bars and it's one of those phenomenon where you just happen to be there at the same time."