You are in the shallow lagoons of 130 million years ago.

The water of the Wealden lagoon was murkier than the open ocean, rich with sediment from the surrounding rivers. You guided your small submersible near a sun-dappled seagrass bed, hoping to spot some dangerous sea monsters lurking in the waters.

Just then, the water broke just ten meters to your right. Emerging from the gloom was a Leptocleidus. It was smaller than you anticipated, barely three meters from its snout to its stubby tail. Unlike the long-necked plesiosaurs of the Jurassic, this creature was compact and streamlined. It moved with a smooth, underwater-flight motion, its four large flippers beating in unison—an efficient, graceful stroke that made it look less like a reptile and more like an underwater bird.

It noticed you. The large, intelligent eyes turned toward the submersible’s viewport. There was no aggression, only curiosity. It held its position, paddling slowly, its body mimicking the movement of a seal.