The stairs to the Grotto (and back up) is a bit of a killer, but the views are well worth it.
The caves itself is interesting and the tour lasts about 45 minutes and was presented in Italian and English. It did however sometimes feel that the Italian story was longer than the English version. You're also able to take pictures (with flash) in the cave - this was the first cave that I visited that allowed this.
The ride to the caves also allows one to visit a panoramic view point and allows you to see the whole bay area at Alghero.
It is a good way to spend a few hours.
PS: you really feel the workout from those stairs the next day!
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