Seismology is one of the few means available to Earth scientists for probing the mechanical structure of the Earth's interior. The advent of modern seismic instrumentation at the end of the 19th century and its installation across the globe was shortly followed by mankind's first general understanding of the Earth's interior: The Croatian seismologist Andrija Mohorovièiæ discovered the crust-mantle boundary in central Europe in 1909, the German Beno Gutenberg determined the radius of the Earth's core in 1913, Great Britian's Sir Harold Jeffreys established its fluid character by 1926, and the Dane Inge Lehman discovered the solid inner core in 1936. It is notable that seismology, even in its earliest days, was an international science. Unlike much of the Earth sciences, seismology has its roots in physics, notably optics (many university seismology programs are, or initially were, attached to meteorology, astronomy, or physics departments), and draws from the literatures of imaging systems and statistical communications theory developed by, or employed in, astronomy, electrical