There were lots of sea-smoothened pieces of shale there, perfect for skipping on the waves, or for decorating a piece of driftwood.

Prrrt.

Gradually, we realized that the mines had been . . . under the sea.

Oh, wow. I knew about the practice of seacoaling, which is more like (arduous) beachcombing: there is a song about it. I had never heard of mining coal under sea.

[edit] I found a song for the miners of Cape Breton.

... and now I think, there is a story out there about the dangers of the mines, and flooded mines, and selkies, and when I have it worked out, I'll share it with you.

I look forward.