Nuclear testing can be monitored using underwater sound. Explosions cause sound waves. These waves will travel through the water when the explosion is in the sea or very near to the sea surface. Sound waves from nuclear explosions have a specific signature sound, and they can be detected by monitoring stations. There are currently 11 hydroacoustic stations in the International Monitoring System. Six of the hydroacoustic stations have underwater microphones called hydrophones that record sounds and transmit them by a cable to a station on land. This is a diagram of the layout of a hydroacoustic station (Provided by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization, http://www.ctbto.org). A subsurface buoy and riser cable hold the hydrophone in the water column where it is able to record underwater sounds.
 

        
      Layout of a hydroacoustic station. Diagram by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization, www.ctbto.org.

      In addition to the underwater hydroacoustic stations, there are also five seismic stations that are on oceanic islands. When a sound wave gets to an island, its energy is changed to a seismic wave (a wave that can move through earth), and can be detected.

      There is a comprehensive nuclear test ban treaty (CTBT) being considered by the United Nations. An important piece of the monitoring is in and above the ocean, which make up over 70% of the world's surface. These areas can be monitored by using sound in the sea.