A "Durabook" - Distributed by Modern Promotions
Copyright 1972 Hanna-Barbera
6.25 x 8 inches, 22 story pages, written by Horace Elias

Each Durabook has two editions: a thick cardboard "Durabook" cover and a hardcover for libraries. There are three different variations of the cardboard cover for "Secret Tunnel." One has a "Durabook" logo near prices of "39¢ / in Canada 49¢." Another version has a "Paul Hamlyn" logo near the price of 39 cents. Paul Hamlyn was a British publisher, but it's still likely this was sold in America because it has the American price. The third variation has the 39/49 prices but no logo.

The backs of the cardboard books have the same artwork as the front except for the price.

Here's the plot: Race is away and Jonny is bored on the island while his dad works on his anti-laser equipment. Jonny sees paratroopers landing on the island, so he runs up the "secret cave" which leads to a trap door in the lab. While trying to leave, Jonny trips and falls on the floor as the invaders burst in. The Quests are held hostage and the leader arrives. Then Race pops in through the trap door, backed by government troops and turns the tables. At supper, Race explains that Jonny fell on the trouble signal button on the floor.

Note: Hadji does not appear and is not mentioned in this story.

The other Jonny Quest Durabooks are: "Jonny Quest and the Lost City" and "Jonny Quest, Adventure with the Salt Plot."

As mentioned above, the hardcover editions were for libraries, so they do not have prices on the covers.

There appear to be two variations of the hardback as well, but just by color. Each has text on the back that reads,