Guust Nolet is a man with quite the job description: he is a professor who manages a fleet of MERMAIDs. These underwater beauties are not folklore but modern-day robotic devices that probe deep within Earth’s mantle – the fiery magma and rock layer beneath the planet's crust. 

The MERMAIDs look a bit like ridged neon cannonballs, each topped with an antenna and a hydrophone, that record seismic waves in an effort to piece together a map of our planet’s interior and the features of its heat budget. Such information is vital to our understanding of why Earth has not cooled over the eons to become a dead, geologically inactive planet like Venus, Mars, or our very own Moon.

Around 70 percent of Earth’s surface is water and much of this area lacks seismic recordings. In fact, the Moon is more explored than the under layers of Earth. That’s why Nolet of Université Côte d’Azur and his research partner Frederik Simons of Princeton University developed MERMAIDs – Mobile Earthquake Recording in Marine Areas by Independent Divers. 

"The initial inspiration came to me in 1989, when I was on sabbatical at the Scripps Institution of Oceanography," Nolet, also an emeritus professor of Princeton University, told IFLScience. "During a lunch conversation with Gerald D’Spain, I learned about his efforts to measure underwater noise using a seismometer aboard a float in the Gulf of Mexico.

At the time, however, the technology was not yet ripe: GPS was just beginning and not miniaturized, while satellite communication was limited to a kilobyte or so using Argos satellites. 

"But when the oceanographers developed floats to measure temperature and salinity, I picked the idea up again. This was 2003."

The team has now expanded its network of nine MERMAIDs to a fleet of 50, in what they say will be one of the most comprehensive seismic mapping systems on Earth. Already their work has paid off: Earlier this year, the team found that volcanoes in the Galapagos are fed via a narrow conduit 1,900 kilometers (1,180 miles) deep that siphons hot rock to the surface, called a magma plume.

"Deployment is very simple and does not require a scientific vessel. It can even be done by hand (they weigh about 50 kg)," said Nolet. "After switching them on and checking that the satellite communication works correctly, one can simply throw them into the water!"

By letting the MERMAIDs descend the depths of the ocean at 1,500 meters (4,920 feet), they can listen to acoustic signals, including those of whales and ships passing. When a MERMAID hears an earthquake, it rises to the surface within 95 minutes and sends by email both a GPS signal and a seismogram of noise unheard to mere human ears. This makes it less labor intensive than a land-based station or an ocean bottom seismometer.

Since waves behave differently based on the material they’re traveling through, scientists can make deductions about what’s below. In this case, the team says they can learn about the temperature, density, and buoyancy of a mantel plume as far down as 700 kilometers (435 miles) beneath Earth's surface. The network of MERMAIDs will specifically clarify the large mantle plume below the South Pacific Ocean by collecting seismic activity from many locations that can be stitched together like one massive scan.

"We actually calculated that a network of 1,000 MERMAIDS, deployed for 5 years worldwide, would allow us to make images of the Earth’s mantle at better than 200 km resolution, enough to answer many open questions about how the Earth actually maintains its temperature and does not become a dead planet like Mars," said Nolet.

Earth has instead remained thermally stable – to the point that we are still able to live on it. As much as volcanoes and earthquakes are deadly nuisances, they are also essential to make our planet habitable.

The team is now working with a second generation that has a life expectancy of 5 to 6 years until the batteries run out, in what Simons said "represents a tremendous improvement over the earlier generation but also over any other autonomous underwater seismic detection system, whether in the water column (where MERMAID is the only player) or at the ocean bottom (the classic "ocean-bottom seismometer, or OBS)."

To combat the environmental impact when the batteries run out, the MERMAIDS have two-way satellite communication so they can relay their GPS position and be picked up by a nearby ship. Twenty-four of the floats are now owned and operated by SUSTech in China, followed by Princeton, JAMSTEC, GeoAzur, Kobe University, and Japanese and French research fleets.

"French company OSEAN is now making our MERMAIDs commercially available – for some 35,000 euros you too, can buy one (and they deliver to an ocean near you!) and then we’ll manage the data collection and interpretation for science," said Simons, who will discuss the work at the 178th Meeting of the Acoustical Society of America.

A limitation of the MERMAIDS is that they can only see S waves for nearby earthquakes, which means the tomographic imaging is for P waves. These waves are somewhat less sensitive to temperature anomalies, which is ultimately the goal of their images. The third generation of MERMAIDs is now in development.  

"Following a philosophy similar to that of smartphones, scientists will be able to program them and attach their own instruments. We expect that this will lead to even more cost savings because costs can be shared between different disciplines: seismologists can work with geochemists measuring CO₂ uptake, biologists observing whale populations, etc," said Nolet.

"Once we go multidisciplinary, the dream of launching ~1,000 MERMAIDS (at a cost of about 60 million over 5 years) will hopefully become a reality."