  The search for gravitational waves is one of today's major scientific
endeavors. A gravitational wave can interact with matter by exciting vibrations
of elastic bodies. Earth itself is a large elastic body whose so-called
normal-mode oscillations ring up when a gravitational wave passes. Therefore,
precise measurement of vibration amplitudes can be used to search for the
elusive gravitational-wave signals. Earth's free oscillations that can be
observed after high-magnitude earthquakes have been studied extensively with
gravimeters and low-frequency seismometers over many decades leading to
invaluable insight into Earth's structure. Making use of our detailed
understanding of Earth's normal modes, numerical models are employed for the
first time to accurately calculate Earth's gravitational-wave response, and
thereby turn a network of sensors that so far has served to improve our
understanding of Earth, into an astrophysical observatory exploring our
Universe. In this article, we constrain the energy density of gravitational
waves to values in the range 0.035 - 0.15 normalized by the critical energy
density of the Universe at frequencies between 0.3mHz and 5mHz, using 10 years
of data from the gravimeter network of the Global Geodynamics Project that
continuously monitors Earth's oscillations. This work is the first step towards
a systematic investigation of the sensitivity of gravimeter networks to
gravitational waves. Further advance in gravimeter technology could improve
sensitivity of these networks and possibly lead to gravitational-wave
detection.
