9.5% of the worlds' gannet population live on this tiny island just 200 meters across. A combination of the expanding colony and droppings on a biblical scale has led to some interesting findings.
The full report can be seen HERE
Apparently Roger Sherman Loomis, an Arthurian scholar, thought that Grassholm (Gresholm) was the location of 'Gwales', the place where Bran's men stayed in the Mabinogion legend 'Branwen the daughter of Llyr'.I don't know how he came to this conclusion as I haven't been able to find his argument yet.
And Bendigeid Vran commanded them that they should cut off his head. "And take you my head," said he, "and bear it even unto the White Mount, in London, and bury it there, with the face towards France. And a long time will you be upon the road. In Harlech you will be feasting seven years, the birds of Rhiannon singing unto you the while. And all that time the head will be to you as pleasant company as it ever was when on my body. And at Gwales in Penvro you will be fourscore years, and you may remain there, and the head with you uncorrupted, until you open the door that looks towards Aber Henvelen, and towards Cornwall. And after you have opened that door, there you may no longer tarry, set forth then to London to bury the head, and go straight forward.
From the Lady Charlotte Guest translation, which you can see at the Sacred Texts Archive, for example.
Peculiar islandish goings-on are not unknown in the vicinity:
I venture to quote from the Pembroke County Guardian. Mr. Ferrar Fenton [..] writes in the issue of Nov. 1, 1896, giving a report which he had received one summer morning from Captain John Evans, since deceased. It is to the effect 'that once when trending up the Channel, and passing Grasholm Island, in what he had always known as deep water, he was surprised to see to windward of him a large tract of land covered with a beautiful green meadow. It was not, however, above water, but just a few feet below, say two or three, so that the grass waved and swam about as the ripple flowed over it, in a most delightful way to the eye, so that as watched it made one feel quite drowsy. You know, he continued, I have heard old people say there is a floating island off there, that sometimes rises to the surface, or nearly, and then sinks down again fathoms deep, so that no one sees it for years, and when nobody expects it comes up again for a while. How it may be, I do not know, but that is what they say.'
From 'Celtic Folklore' by John Rhys. A cynic might put it down to all the fertiliser coming off the island. But that's not terribly romantic.
Grassholm might be tiny but it has thousands of noisy stinky gannets on it: about 10% of the world's population (I can vouch for its impressiveness myself). Consequently you can't set foot on it, but it would seem intrepid solitude-seeking prehistoric people did, as Iron Age pottery has been found there. Presumeably under a pile of gannet poo. Perhaps the gannets hadn't taken up residence at the time. Or perhaps they had (what's for tea, mum? Omlette with gannet and fish. What, not Again??)The island is miles away from any others and about 10 from the mainland.