Bedrock is the solid layer of rock that lies beneath all the loose soil, sand, and gravel on the Earth's surface. If you could dig down through your backyard deep enough, past all the dirt and clay, you'd eventually hit bedrock: the hard, unbroken rock that forms the foundation of the ground beneath your feet.

Geologists study bedrock to understand what's really going on underground. In some places, like the cliffs of the Grand Canyon, you can see bedrock exposed on the surface. In other places, it lies hundreds of feet down. Cities like New York were built where they were partly because bedrock sits close to the surface, providing a strong foundation for enormous skyscrapers.

The word also means a fundamental principle or basic fact that everything else depends on. The bedrock of mathematics includes basic operations like addition and multiplication: you need to understand those before you can do algebra. When someone talks about the bedrock principles of democracy, they mean the core ideas that hold the whole system together, like the rule of law and free elections. Just as geological bedrock supports everything above it, a bedrock principle is so fundamental that other ideas are built upon it.