b30w0lf writes "Gravitational wave detection — i.e. the detection of propagating ripples in spacetime — is a hot subject these days, with ground-based interferometer experiments like LIGO active, and hopes for a space interferometer like LISA. However, physicist Freeman Dyson proposed back in 1969 that the earth itself could be used as a gravitational wave detector. The idea is behind the approach is that gravitational waves impact the earth's crust, causing potentially detectable seismic waves. Using Dyson's approach, Physicists at Harvard and NINP, Florence were able to put an upper limit on the intensity of gravitational background radiation based on a year of observational seismic data (abstract, full pre-print). The upper limit they found improved currently laboratory upper limits by 9 orders of magnitude."