A boom or a chain (also boom defense, harbor chain, river chain, chain boom, boom chain, or variants) is an obstacle strung across a navigable stretch of water to control or block navigation.
Spill booms are temporary floating barriers used to contain marine spills, protect the environment, and assist in recovery. A boom includes a containment partition that floats on and extends above the water's surface and a "skirt" or "curtain" that sinks into the water. Booms are floating, physical barriers to oil, made of plastic, metal, or other materials, which slow the spread of oil and keep it contained.
Oil is less dense than water, giving it buoyant properties. That means when oil is released into the ocean, it naturally floats to the surface rather than sink to the bottom or hover in the middle. This makes containment booms highly effective at creating a barrier that traps oil in a thick layer on the water's surface.