Beneath the Hill (Abaloc Book 1) - Jane Louise Curry - Bog

When Miggle creates a make-believe treasure hunt, she never dreams it could turn into a very real and dangerous battle against an ancient evil-or that nearby strip-mining could be a threat not only to the landscape but also to the existence of the Fair Folk who dwell within the mountain. In the end, it's up to Miggle and her siblings and cousins to help these mysterious and beautiful people find their way to safety.