Find out about secret tunnels past and present. Secret bunkers, underground cities and war-time tunnels - it's all in here. Then read a story about treasure, a secret tunnel and a secret exit. This book is part of "Thunderbolts" a series of 24 levelled reading books, covering popular non-fiction topics. The 24 low level, full-colour titles offer progression over four levels and have particular appeal to older readers. Each book features a non-fiction section together with a fully illustrated fiction story on the same topic.