>Note that the 'bad soil for digging' would be an excellent cover for underground tunnels.
Sorry to poke holes in your silly tunnel theory, but the islands off the coast of Washington are very rocky. I have built decks out there. Mostly you cannot dig holes for concrete. You have to chip away the stone so that it is level and bolt the posts to the rock with an anchoring system.
There is very little possibility of tunnels, if there were, they would be below sea level and flooded. Most of our smaller islands are not very tall around here.
Here's some geology sauce: IGNEOUS ROCKS
https://www.nps.gov/parkhistory/online_books/geology/publications/state/wa/uw-1927-2/sec4.htm