Under the Mountain — Maurice Gee

Beneath the extinct volcanoes surrounding the city, giant creatures are waking from a spellbound sleep that has lasted thousands of years. Their goal is the destruction of the world. Rachel and Theo Matheson are twins. Apart from having red hair, there is nothing remarkable about them - or so they think. They are horrified to discover that they have a strange and awesome destiny. Only the Matheson twins can save the world from the terror of what is under the mountain.