The arrival of a crate of pineapples and an odd pet at the Hollister home leads the family on a Puerto Rican vacation.

An abandoned rock quarry, mysterious flying objects, and a valuable deposit of titanium lead the five Hollister children--Pete, Pam, Ricky, Holly, and Sue--into their latest thrilling adventure. When a package of instruments from a weather balloon is parachuted onto their front lawn, the Hollisters are baffled and very excited. Just what is the meaning of the orange parachute and its dangling white box? The Hollisters connect the balloon with the strange lights which have been appearing at night over their Shoreham home, and once again they start off on a trail of detection. A chance visit to a local "rock hound," a man who collects minerals, proves to be the clue to the puzzle. Then their detective work takes them to a fantastic, spooky rock quarry which turns out to be the location of a hidden laboratory for weather experiments and also the scene of an attempt to steal some valuable titanium. As always, the Hollisters have several breath-taking experiences along the way to solving their mystery. Pam accidentally falls into the cold, green waters of the quarry pool where a monster is said to live! Pete and Ricky inspect an old-fashioned open cockpit airplane and suddenly find themselves flying in the sky without a pilot--without knowing how to land! As the Castle Rock mystery thickens, the Hollisters make an unexpected visit to New York where they see their old friend Hootnanny Gandy who helps them get to the bottom of another fascinating adventure. Paperback; 170 pages with 20 illustrations.

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