For one of Europe's most amazing marine wonders, head to Sardinia's Grotta di Nettuno.

The caves can be reached by boat or down a steep cliff-side staircase of 656 steps. From there, follow the natural entrance to a vast internal salt lake stretching 120 metres across.

Continuous sea movements in the caves have formed mysterious chambers and caverns around the lake but the water feature itself is worth a look for the unusual columns rising out of it.

In the 18th century the complex was discovered and used by local fishermen. It was also the home of the rare monk seal, now extinct.

All visits to the Grotta are part of hourly tours lasting approximately 45 minutes. They include English and Italian commentary and occasionally French and German translation too.

For other sightseeing ideas in Sardinia, pay a visit to Alghero's medieval walled centre and splash out on a horse-drawn carriage tour of its picturesque narrow streets.