To skin-dive means to swim underwater while holding your breath, without using scuba tanks or other heavy breathing equipment. Skin-divers wear a mask to see clearly underwater, fins to help them swim faster and deeper, and often a snorkel (a J-shaped tube that lets you breathe while floating face-down on the surface). When they want to explore deeper, they take a big breath, dive down, look around at fish or coral or whatever interests them, then swim back up for air.

Unlike scuba divers who can stay underwater for an hour breathing from tanks, skin-divers must surface every minute or two for fresh air. Some skilled skin-divers can hold their breath for several minutes and dive surprisingly deep, but most people start by diving just ten or fifteen feet down in a swimming pool or calm bay.

Skin-diving takes practice and training to do safely. You learn to equalize the pressure in your ears as you descend, conserve energy underwater, and follow safety rules. Many people start skin-diving as kids in swimming lessons, then continue as adults because it's an exciting way to explore the underwater world without complicated equipment.