The discovery, and its announcement last week, has prompted a race against time, with archaeologists keen to retrieve the fragile contents of the tomb before they are destroyed by the water filling one of the chambers.
The suspected tomb was uncovered on September 18, by Hamish Mowatt, by the car park of the Skerries Bistro, at Banks, South Ronaldsay.
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 – 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.”
Mr Mowatt and his fiancee, Carole Fletcher, who owns and runs the bistro, contacted county archaeologist Julie Gibson, who confirmed they had a Stone Age chambered tomb on their property.
Although it is difficult to tell at present, the structure may contain a number of chambers, one of which has been cut down into rock and is capped by a massive flat stone.
Mrs Gibson said: “The chamber under the huge stone is made up of three rock cut walls, some with supplementary building, and a fourth built wall, which has a lintel in it.
“We can see that there are at least three skulls inside, and possibly pottery, which need to be recovered before the water inside the structure destroy them.
“When it comes to excavating, the large stone ‘roof’ needs to be handled carefully and we need to see whether we can safely enter the chamber leaving the rock in place or whether it has to come off first.”
Aside from the Crantit tomb in 1998, which yielded human remains in a poor state of preservation, the Banks tomb is the first Neolithic cairn which contains human remains since the excavation at Quanterness in the early 1970s.
“It’s a very exciting find,” said Mrs Gibson. “Not only do we have the discovery of relatively undisturbed human remains, but we’ve now got two tombs – the Banks tomb and the nearby Tomb of the Eagles – in close proximity and both found in relatively recent times, where we can see how the dead were being handled in the Neolithic. The possibility of untouched human remains is particularly exciting, considering the techniques we have now that were not available back in the 1970s.
“Because of the construction of the tomb, we need to think carefully about how it is excavated and I hope that Historic Scotland will be able to help in that respect.”