Mr Mowatt said he had always wondered what lay under an 8ft stone in the garden and eventually curiosity got the better of him.
He dug a small hole close to the stone to see how thick it was. He then managed to get a thin wire pushed under the stone and confirmed there was definitely a space underneath. While doing this, a finger-hole size appeared in the earth to his right. This allowed him to push the wire in â€” to a depth of three feet.
By carefully removing a small area of earth and two stones, Mr Mowatt could see a rock face. Shining a torch inside, he saw a chamber with about nine inches of water lying in the bottom.
Mr Mowatt added: "I have an underwater camera, so I got it in through the hole and the monitor rigged up. On the screen, I could see the rock face clearly, but when I went further I could clearly see what I thought was a white skull, with two eye sockets, looking back at me."
So far, three skulls are visible in the stone chamber, which is filling up with water. Experts think there might be multiple connected chambers on the site. http://www.orkneyjar.com/archaeology/bankstomb2010.htm -via TYWKIWDBI
(Image credit: Sigurd Towrie)