" 'This is easy,' Sam said. 'I always thought it would be much harder than this to dig a cave. It's almost as if it's digging itself.' Margaret stared at the hollow under the tree. 'Sam,' she whispered, 'look! It IS digging itself.' They couldn't see any dirt piling up, but the cave was getting bigger all the time."
Used availability for Ruth Chew's The Secret Tree House
November 1975 : USA Paperback
Title: Secret Tree House
Author(s): Ruth Chew
ISBN: 0-590-09945-0 / 978-0-590-09945-5 (USA edition)
Publisher: Scholastic Paperbacks