A submarine is a watercraft designed to travel underwater. Unlike regular boats that float on the surface, submarines can sink beneath the waves and cruise through the ocean depths, then return to the surface when needed.

Submarines work by controlling their weight and buoyancy. They have special tanks that can fill with water to make the submarine heavy enough to sink, or fill with air to make it light enough to rise. It's similar to how you can sink in a pool by letting air out of your lungs, or float by taking a deep breath.

Military submarines can stay underwater for months at a time, allowing navies to patrol the oceans secretly. These submarines often carry advanced weapons and surveillance equipment. Research submarines help scientists explore the deep ocean, discovering new species and studying underwater volcanoes and trenches that humans could never reach otherwise.

Submarines represent one of humanity's boldest engineering achievements: creating vessels that can withstand the crushing pressure of deep water while keeping crews safe inside. Modern nuclear-powered submarines can even make their own air and fresh water, creating a self-contained world beneath the waves.