The Romulan prototype interphase generator
An interphase generator was an experimental type of cloaking device developed by the Klingon Empire in the 24th century. By combining a cloaking device with a molecular phase inverter, which could change the structure of matter to pass through normal matter and energy, it would give a vessel the ability to hide anywhere, even inside a planet. Additionally, such a "phased" vessel would be immune to convential weapons. The Klingons eventually abandoned this research due to several accidents which resulted from these experiments.
In 2368, the Romulans attempted to develop their own interphase generator and install it aboard a science vessel, but inadvertantly caused a forced chamber explosion in their resonator coil, severely damaging the ship. The USS Enterprise-D answered the Romulans' distress call, and assisted them in making repairs. Upon beaming back to the Enterpise, however, Lieutenant Commander Geordi La Forge and Ensign Ro Laren became contaminated with chronitons leaking from the Romulans' cloaking device, and became phased themselves, unable to interact or communicate with anyone but each other. La Forge and Ro were eventually de-phased when they were subjected to a high-intensity anyon emission. (TNG: "The Next Phase")