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.