Kaz B.
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To get to Mykines you must catch a ferry from Sørvágur which departs only twice a day and can be impacted by the very wild and changeable weather. Ferry tickets must be purchased in advance and the availability is seasonal.
You must also pay for a compulsory guide - a fee of 250DK when we did it and the group size is about 15-20 people. You can book a private guide if you wish.
As for the Island. The scenery is spectacular and the hike is fairly easy (although the footing can be quite slippery). The weather changes very quickly so be prepared. Appropriate boots, jacket, hat and yes, even sun protection could all be need in the space of your short visit.
You can no longer hike to the Lighthouse du to a rather substantial landslide which destroyed the path. The hike you can take offers some incredible views. Even though we were there in May, we did not see any Puffins up close. The noise from the group we were with and their inability to simply stay back and be quiet had more to do with that than the absence of Puffins. They are very shy and easily scared off.
You will see people complaining about this fee, but as with other things in the Faroe Islands this is actually pretty reasonable as the island has a very short season and we are in fact entering a very delicate and private environment.