Part of the popular "Radio-Phone Boys" series, featuring mystery and technology of the era. Centers on adventures involving a cutting-edge (for the time) submersible vehicle and blends science-fiction elements with typical boyhood exploration and mystery. Bound in faded red cloth over boards with black stamping on front cover and spine, cloth chipped at head and foot. Bumped along all edges. DJ in POOR condition with several closed and open tears and liquid staining along spine, now in mylar. Previous owner’s stamp on first free endpaper. Foxed.