Sunday afternoon I visited a well known cairn field in RI. The cairns are compactly constructed, tall and rounded. Although I have been here many times, there is always something new revealed by changes in the light. As I walked along, I noticed this cairn. with a vertical opening extending through three layers of rocks. The late afternoon sunlight was playing on something inside the cairn.
Other cairns also seem to have central depressions. Cairns with dimples or vaults often appear on the Rockpiles and Ceremonial Landscapes blogs.
Maybe the cairns were originally constructed with cavities, and then the cairns were vandalized, or simply fell in. Cairns are often opened due to stories that there is "treasure" in cairns. The treasure is simply the arrangement of the stones.