A fully grown mosasaur attacks a swimming pool lifeguard and kills him. It then returned through the Anomaly that brought it there when the chlorine in the water began irritating its skin. It then appeared at a reservoir in west London and regurgitates a bolus of the undigested remains of the lifeguard. Connor and Abby who had waded out into the reservoir to take water samples where then stalked by the creature. They then rush onto land but are stopped by a high metal fence. Another smaller mosasur comes ashore after them and Connor fights it off with an oar. It backs off into the water returns through the anomaly just in time as it closes.

It was last seen when Nick Cutter had travelled through the anomaly looking for his wife Helen. He fires a spear at its head, but its skin is too thick to penetrate. It attacks him but only bites down on his scuba, which while leaving him unwounded, also leaves him without air. As the mosasaur swings round for its final attack, the adult mosasaur attacks it, killing it almost instantly and swimming off with its kill. It is unknown what type of mosasaur it is, but it's likely to have been Tylosaurus, the biggest known kind.
1 3 mosasaur in pool