You are in the forests of North America, 75 million years ago.

The air in the Cretaceous was thick, smelling of wet ferns and sulfur. You were exploring the forest, when you hear a loud sound. It was not a scream, but an unsettling, low-frequency boom—an eerie, trumpet-like bellow that vibrated through the mud beneath your feet and resonated in your own chest. But curiosity drives you to investigate the source of these strange alien calls.

Pushing past a massive fern, you stop, frozen. Before you, in a clearing, is a massive herd of Lambeosaurus. They are magnificent, standing on their hind legs to browse on foliage before dropping to all fours to chew on conifers.

The sound comes again, shaking the very air. A particularly large adult—perhaps 30 feet long with a towering, hatchet-shaped crest—opens its duck-like bill. It doesn’t roar; it bellows. The air passes through those hollow, hatchet-like crests, turning the call into a deep, booming trumpet sound that carries for miles through the dense, swampy, river-cut forest.

The sound is unnervingly similar to a modern cassowary, but magnified a hundred times. As you watch, another Lambeosaurus answers back with a slightly higher pitch, the crests acting like a resonating wind instrument to coordinate the massive herd. They aren't screaming in fear; they are talking. A deep, vibrating, prehistoric conversation that makes the ground tremble beneath your feet.