Underwater means beneath the surface of water, in the space where water covers everything above. When you dive into a pool and open your eyes, you're seeing the underwater world: the blue-green light, the distorted shapes, maybe someone's legs kicking nearby.

Entire ecosystems exist underwater. Coral reefs thrive in the ocean, fish swim in underwater caves, and kelp forests sway with the currents. Scientists use special submarines to explore underwater mountains and valleys on the ocean floor. Archaeologists discover underwater ruins of ancient cities. Some animals, like whales and sea turtles, spend much of their lives underwater, surfacing to breathe.

The word also describes things designed to work beneath the surface. Underwater cameras can film the ocean depths without getting ruined. Underwater microphones, called hydrophones, let researchers hear whale songs. Navy divers wear underwater equipment to repair ships below the waterline.

Sometimes people use underwater as a metaphor: when someone's in serious financial trouble, owing more money than their home is worth, people might say they're underwater on a loan or mortgage. They're drowning in debt, so to speak, struggling to get their head above water.