Great Structures - Drop tubes: creating weightlessness for scientific research on Earth

In physics and materials science, researchers use drop tubes, also known as drop towers, to produce controlled periods of weightlessness for objects under study. During an experiment, payloads in free fall are halted using air bags, polystyrene pellets, or mechanical and magnetic brakes. In other setups, high-speed impacts with a base substrate are deliberately studied. While many facilities are above-ground towers, some operate underground, such as NASA Glenn's Zero Gravity Research Facility, which utilizes a vertical shaft extending 510 feet below ground level.

Source: Wikipedia