Enid Blyton 
Five Go Down to the Sea 
Famous Five #12

When the Five go to spend their holidays at Tremannon Farm on the Cornish Coast, they learn about the location’s incredible past concerning some evil examples of smuggling. The peacefulness and tranquillity of the area now suggests that these days are well and truly gone, but then one stormy night, the lights appear again, and Tremannon’s dreadful past comes flooding back.