An eon is an incredibly long period of time, so vast that it's hard for our minds to grasp. In everyday conversation, when someone says “I haven't seen you in eons,” they're exaggerating to mean it's been a very long time, maybe just a few months or years.

But in geology and science, an eon means something specific and almost unimaginably long: hundreds of millions or even billions of years. Scientists divide Earth's 4.5-billion-year history into just four eons. The current eon, called the Phanerozoic, began about 540 million years ago when complex life started appearing in the oceans. The eon before that lasted over 4 billion years.

To put this in perspective, all of recorded human history, from the first writing systems to today, covers only about 5,000 years. An eon is roughly one million times longer than that. Dinosaurs lived and went extinct, mountains rose and eroded away, and continents drifted across the globe, all within single eons. When you hold a rock formed an eon ago, you're touching something that existed long before almost anything you'd recognize on Earth today.