In the last few decades seismologists have mapped out elastic
wavespeeds of the Earth's interior with often perplexing if not always
uncontested detail. Earthquake sources used in seismic tomography lie
mostly on plate boundaries; receivers mostly on dry land. The uneven
coverage resulting from this fundamentally inadequate source-station
distribution leaves large volumes inside the Earth entirely unsampled.
Placing seismic stations on the ocean bottom is often touted as the
only solution. The MERMAID project (Mobile Earthquake Recorder in
Marine Areas by Independent Divers) is a radical low-cost alternative
that uses passively drifting autonomous hydrophones with a now proven
potential to record hundreds of distant earthquakes over their
projected life span.