You are in the forested plains of North America, 150 million years ago.

The ground didn't shake at first; it hummed. A low, rhythmic vibration that you felt in your teeth before you heard it. You pushed through the thick fern canopy and froze. The trees ahead didn't look like trees anymore—they were legs. Massive, pillar-like legs, gray and textured like ancient stone. You tilted your head back, higher and higher, until your neck ached, following the impossible length of a neck that seemed to scrape the clouds. It was a Supersaurus, and it was eating the very tops of the conifers, completely indifferent to the tiny creature that is you watching from the mud below. The sheer volume of the animal was staggering; it was less a creature and more a living mountain moving through the forest.