You are in the oceans of Europe, 155 million years ago.

The water was a murky, warm green, characteristic of the shallow Jurassic seas. You were drifting near the surface, watching a school of small fish darting around when the sunlight above you suddenly disappeared.

You looked up, expecting a storm cloud, but it was alive.

Leedsichthys was massive—larger than any shark or whale you had ever imagined, stretching at least 15 meters from its rounded snout to its thrashing tail fin. It moved with a slow, hypnotic grace, a giant filter-feeder unbothered by its smaller surroundings. Its mouth was open, a cavernous dark hole, swimming slowly to force plankton through the specialized gill rakers hidden behind its enormous gills.