The advances made using data from the sparse land-based digital seismic networks operating since the mid-1970s have been spectacular. There are now tomographic maps of the Earth's interior, reaching from the lithosphere to the inner core of the Earth. A clear potential has been demonstrated for the mapping of temperature anomalies and flow in the mantle by the determination of velocity anomalies and changes in azimuthal or polarization anisotropy. The first maps of the topography of the core mantle boundary were published last year. However, the resolution of these studies and the uniqueness of their results is limited by the lack of adequate spatial sampling of the seismic wave field over the entire globe.