Tom Clancy is the author whose name is most often associated with the video game series Ghost Recon, The Division and Splinter Cell. But his name became popular after publishing the book The Hunt for Red October in 1984 which was later adapted into a movie starring Sean Connery. In this book a Red October nuclear submarine is hunted but difficult to track because it uses a silent magnetohydrodynamics (MHD) propulsion system.
Many don't realize MHD technology has been around for over seven decades but size constraints and energy efficiency keep it from being widely used. However, DARPA, an agency that encourages new technology users in the United States, has started the Principles of Undersea Magnetohydrodynamic Pumps (PUMP) program to realize MHD technology for ships and even submarines for military use.
The MHD system produces thrust through an electromagnetic system. The advantage of MHD is that it does not have any moving mechanical components. A magnetic field is used to push water into a tube. Imagine a jet engine but without a compressor system, fuel to burn or a spinning fan.
From a military point of view, the MHD is a silent propulsion system. When used on submarines such as "Red October", they cannot be detected by current hydrophone systems. For the purpose of spying and invading enemy waters, MHD will produce submarines that have the same capabilities as "stealth" aircraft that cannot be detected by radar.