The vehicles are modular, composed of multiple genetically engineered animals with the ability to clip together with specialized structures. The vehicles appear designed with a humanoid size in mind, and the cockpits can be used by humans. The vehicles have their own will (with the brain in the cockpit), but this can be overrided by the controls in the cockpit. There are also vehicles with a large brain in place of a cockpit. These cannot be overrided. A wide variety of tools were made for the vehicles. The society that made this technology is now extinct, and has been for multiple generations of the vehicles
What scenario could lead to these vehicles not being discovered until modern times?