The return of French research vessel Atalante to Nouméa, New Caledonia end of August 2019, will concluded the deployment of 28 floating seismometers, marking the completion of a new array of 51 such instruments. The robotic sensors will drift in the Pacific Ocean for up to five years, recording seismic waves from earthquakes worldwide, and transmitting them in near real-time. Scientists from China, France, Japan, and the United States, led by Chief Scientist Yann Hello (Géoazur, Sophia Antipolis, France) made the team on board the Atalante (Figs. 1 and 2).

Twenty-four of the 28 new robotic floats were provided by the Southern University of Science and Technology (SUSTech, Shenzen, China), and 4 by Kobe University and JAMSTEC (Japan). Four earlier campaigns, conducted in 2018 and 2019 by the French research vessel Alis and the Japanese Mirai (Fig. 3), launched instruments owned by Princeton University (16), Kobe/JAMSTEC (5) and Géoazur (2).

The array forms the backbone of project SPPIM (South Pacific Plume Imaging and Modeling), an international collaboration to image, in detail, the massive mantle plume in the lower mantle beneath the South Pacific. The lower-mantle root of this "superplume" produces a stronger signature in global seismic imaging than the mantle plume underlying Hawaii, but the details of its anomalous density and temperature structure, how it relates to the surrounding mantle, and why - unlike smaller mantle plumes - it may have existed for much of geological time, remain to be worked out. The Pacific superplume appears connected to a group of upper-mantle hotspots in the South Pacific but does not itself seem to rise all the way to the surface. Gaining insight about its buoyancy and viscosity, density and temperature, requires precise measurements of the propagation speeds of seismic waves traversing the plume. Understanding why the plume may experience difficulties reaching Earth's surface demands sharp seismic images, especially at the depths below 600-700 km, a potential barrier for material flowing from the lower into the upper mantle. Over the course of the last few decades, various projects using seismometers placed on¿and recovered from¿the ocean bottom have yielded tantalizing clues but blurry images.  Long-lived, non-recovered, autonomous drifting sensors, such as the newly deployed MERMAIDs (Mobile Earthquake Recording in Marine Areas by Independent Divers), are fulfilling the promise of creating, over time, the dense and wide data set of seismic observations needed to ¿cover¿ the Earth below us from many angles, much like sensors would rotate around a human body in a hospital CAT scan. MERMAID is a revolutionary submarine ¿drone¿ with room for up to 8 sensors, originally conceived by Frederik Simons and Guust Nolet at Princeton University. The current (third-generation) MERMAID (Fig. 4) was developed by Yann Hello at Géoazur (Hello et al., AGU, 2011) in collaboration with Osean Underwater Technology in France with the support of the European Research Council project GlobalSeis led by Guust Nolet. It is a freely drifting diver that combines 1) a hydrophone to detect earthquakes under water, 2) GPS for location and timing accuracy, 3) an on-board digitizing and processing unit that uses wavelet detection and discrimination algorithms, and 4) an Iridium satellite unit for near real-time data transfer with two-way communication. The instrument, 50 kg in air, submersible to 4000 m water depth, with a projected lifetime of up to 5 years, is now commercially available from OSEAN (info@osean.fr). SPPIM is the first field campaign of EarthScopeOceans (www.earthscopeoceans.org), aninternational program that aims to build a global network of floating seismic arrays in the world's oceans.