Pacific Vortex!

Dirk Pitt finds a message capsule from a missing nuclear submarine while on vacation in Hawaii and is immediately dragged into a mission involving a fog-shrouded stretch of ocean where ships vanish without explanation. Clive Cussler's formula — historical intrigue, technical lore, impossible odds — is already visible here, and the underwater mystery gives the book a genuinely distinctive hook. The honest caveat is that this was written first but published sixth in the series, and it shows: Pitt feels more brooding than his usual wisecracking self, and the romantic subplot with the dangerous Summer Moran ends without real resolution. Worth reading as a series artifact, but not the best entry point. Moderate action violence; fine for adults and older teens.