Nuclear explosion are extremely strong sources of underwater sound. It is likely that past tests had
significant impacts on marine mammals in the vicinity of the test sites. No monitoring data are available,
however, to document impacts of nuclear tests on marine mammals. To ensure compliance with the test
ban treaty, an international monitoring system is being implemented, including a series of hydrophone- and
island-based seismic stations to detect high-intensity sounds (www.ctbto.org). This information is
transmitted, in real time, to the International Data Centre where analysts evaluate the data for indications of
nuclear explosions. Physical characteristics of the oceans allow the sounds of such explosions to travel for
extremely long distances with little energy loss, and monitoring is conducted over a large fraction of the
world’s oceans with a small number of stations. The network designed for ocean monitoring contains 11
stations located primarily in the Southern Hemisphere.