On a hill near Veryan is a barrow, in which Gerennius, a mythical king of Cornwall, was said to have been buried many centuries ago, with his crown on his head, lying in his golden boat with silver oars. It was opened in 1855 when nothing but a kistvaen (a rude stone chest) containing his ashes was found. His palace of Dingerein was in the neighbouring village of Gerrans. A subterranean passage, now known as Mermaid’s Hole, one day discovered when ploughing a field, was supposed to have led from it to the sea.
p30 in
Cornish Folk-Lore
M. A. Courtney
The Folk-Lore Journal, Vol. 5, No. 1. (1887), pp. 14-61.
I can’t see Mermaid’s Hole on the map, so be careful not to fall into it if it’s still there.