First find out what is there

 kw: book reviews, nonfiction, oceanography, cartography, mini-biographies

Exploration is mainly finding out what is "out there" and the most useful product of exploration is a map or chart.

Here we see mapper Cassie Bongiovanni with a color-coded contour map of the Molloy Hole in the Arctic Ocean, pointing out the deepest location within it. That spot, at a depth of 18,212 feet or 5,551 meters, is the deepest point in the Arctic Ocean, which is the only ocean without a trench that reaches into the Hadal Zone, which is defined to be deeper than 6,000 m (19,685 ft).

Ms Bongiovanni and a rotating crew of helpers accompanied Victor Vescovo on his ship DSSV Pressure Drop during the Five Deeps expedition (I reviewed another book about it in 2021). In the next image she is shown at the ship's mapping station with Aileen Bohan. Over the year-and-a-half of the Five Deeps expedition sponsored and led by Mr. Vescovo she mapped the deepest points in the five oceans and large surrounding areas, a total area exceeding that of Texas. The ship had a million-dollar multibeam sonar attached, which was needed to pinpoint the deepest spots, and to provide high-resolution mapping. The resulting maps were submitted to the Seabed 2030 Project.

When I began reading The Deepest Map: The High-Stakes Race to Chart the World's Oceans by Laura Trethewey, I naturally assumed that the focus would be the Seabed 2030 Project. It soon became clear that a major aim of the book is sharing the story of Cassie Bongiovanni, who was just 25, a freshly-minted mapper, when she got the opportunity to be the on-board mapper for the Five Deeps expedition and, as a critical side benefit, prepare detailed maps of several percent of the ocean floor.

It has been said, very frequently!, that we know the surface of the Moon in greater detail than we know the sea floor. Let's think about that a moment. With a 16-inch (40 cm) diameter telescope, visual acuity during moments of the best seeing can be 1/3 of a second of arc, or about 1/620,000 of one's distance to an object. The Moon's distance at closest approach (which occurs monthly) is about 360,000 km. Over time, a dedicated visual observer can get acquainted with the features of the visible side of the moon with a resolution of about 580 meters, or 1,900 feet. Of course, space telescopes and spacecraft have mapped the Moon with much more detail than this. But consider: Right now, we are about halfway through the Seabed 2030 Project, and about 1/4 of the total area of the ocean bottom has been mapped with resolution of 1,000 meters or better. A visual astronomer with a biggish backyard telescope does indeed see the Moon better than we see the ocean floor! The initial goal of Seabed 2030 is 100 m resolution over most of the ocean, and 800 m resolution in the deepest depths. At that point, there will still be parts of the ocean that are less known than the Moon can be known by a visual astronomer.

Once you have a map, what do you do with it? For seafaring purposes, a specialized map called a chart facilitates navigation. On shipboard, "chart" has a special meaning; nautical charts show tons of information, including all the named places known at that time, information about currents and depths that are known, ferry routes, plus navigation aids such as LORAN and, of course, latitude and longitude lines. Near coastlines, the depth indications are crucial; you don't want to run aground.

In the deeper ocean, other business interests predominate. An enormous debate going on right now is what to do about metallic deposits on the seabed, such as "manganese nodules" or "polymetallic seabed nodules". What are these nodules?

Large areas of the sea floor, typically at depths between 2 and 5 km, known as the Abyssal Plain, are littered with, and sometimes almost solidly covered with lumps about the size of a potato, consisting of sulfides and oxides of several metallic elements, such as manganese, nickel, cobalt, and some "rare earths" (which aren't rare, just hard to separate from one another; there are 17 such elements that are chemically very, very similar). Mining companies are just champing at the bit to go out and dredge the sea floor, to hoover up millions of tons of the nodules and refine them for the valuable metals they contain.

Their opponents are environmental and scientific interests, saying, "Let's find out what is there first, before destroying it." The nodules grow slowly, gaining at most an inch in size per million years. The oldest parts of the sea floor are about 200 million years, because the marine plates are pulled down into the trenches and recycled. Depending on where one finds it, a nodule the size of your fist could have taken between five and fifty million years to grow to that size.

Living things dwell on and near them. The shrimp in this picture isn't here on a sightseeing expedition. It expects to find something to eat.

Deep water corals, sponges and other animals live on the nodules, where they can have a firm attachment. The surrounding seabed is soft mud, to which such creatures cannot attach. But burrowing animals live in the mud, where they might be preyed upon, and the nodules provide a modicum of safety compared to an open, flat muddy field.

Ms Trethewey attended meetings of the International Seabed Authority, where such issues are debated. Later she asked Victor Vescovo of his opinion. He has several reasons to say that seabed mining will not prove economical. The most important is the law of supply and demand. A sudden increase in supply will drive prices down. Secondly, it is easy to lose stuff at sea. If a cable breaks and your 50-ton mining machine falls to the seafloor untethered, it will cost more to retrieve it than it did to build it, but building its replacement will take months (years?).

Thanks to Mr. Vescovo and Ms Bongiovanni, we have great maps of the greatest depths. May the mapmaking continue and prosper!

The book covers a lot more ground than the tidbits that interested me, that I've riffed on here. A great read!