The Alcantara is a river in Sicily and it is 52 kms long. Several thousand years ago, the river bed was blocked by a lava flow from Mount Etna. As the lava was cooled much more quickly by the water than it would have done otherwise, it crystallised in the form of columns. Over the next millennia, the river naturally eroded a channel through these columns, resulting in impressive gorges and ravines such as the "Gole dell' Alcantara.
I advise taking a wet suit if one wants to enter the water for it is almost freezing. If one is not of the adventurous type the restaurants and bars in the actual locality make this visit a really worthwhile trip and experience that stays with you long after the visit is over. I went there overland from Malta but a car to get there is highly recommended.
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