The Island Of Adventure (Revised) by Enid Blyton
For Philip, Dinah, Lucy-Ann and Jack, the holiday in Cornwall is everything they'd hope for - until they begin to realize that something very sinister is taking place on the mysterious Isle of Gloom.
But they're not prepared for the dangerous adventure that awaits them in the abandoned copper mines and secret tunnels beneath the sea.
Other titles in the series:
The Castle of Adventure
The Valley of Adventure
The Sea of Adventure
The Mountain of Adventure
The Ship of Adventure
The Circus of Adventure
The River or Adventure