The Geography and Map Division is buzzing with anticipation for this Thursday’s GIS Day-themed Live! At the Library event on ocean mapping, and I’ve been channeling my excitement by reading Laura Trethewey’s recently published book, The Deepest Map: The High-Stakes Race to Chart the World’s Oceans. It led me to learn about the Puerto Rico Trench, which is a long, flat subduction zone about 100 miles north of Puerto Rico, where the North American and Caribbean tectonic plates meet and scrape past each other in a massive transform fault. At their meeting point, the heavier North American plate subducts below the Caribbean plate, creating a chasm at which one can find the deepest point of the Atlantic Ocean: the Milwaukee Deep. To my delight, I discovered while searching the stacks one day that we have an early – possibly the earliest extant – U.S. Hydrographic Office map that shows the Puerto Rico Trench with the Milwaukee Deep included.
As The Deepest Map points out early on, one reason that ocean floor mapping remains a perennial hot topic is that we just aren’t anywhere close to being done with it. An often-repeated phrase people use when discussing seabed mapping, and which is dissected in the book by scientists who both love and hate it, is that we have mapped the surface of the Moon in far more depth than we’ve mapped our own planet. (One quoted scientist who hates this phrase points out that the Moon is only about as wide as Australia, so in that person’s mind it’s not a fair comparison.) However, the point remains: the sea floor is a major blind spot in Earth mapping, still very little understood. Even now in 2024, according to the Seabed 2030 Project – a joint project between GEBCO and the Nippon Foundation to map the ocean floor completely by 2030 – only 26.1% of the ocean has been mapped at a resolution comparable to what we would expect for maps of the Earth’s land surfaces. (Our GIS Day keynote speaker, Dr. Vicki Ferrini, is Head of the Seabed 2030 Project’s Atlantic and Indian Ocean Center!)
Measuring coastal sea and ocean depths goes back to ancient times, when sailors dropped marked ropes attached to lead weights and recorded the results, usually to ensure safe entry into harbors and other routine needs. Measuring far greater ocean depths, away from coastlines, kicked off around the middle of the 19th century as scientific and military interest in doing so combined with new mechanized sounding machines that used wire and extremely heavy weights, so that measurements could be taken with more standardization and less reliance on human intervention. In the 20th century, sonar replaced wire and weights for deep ocean soundings, making the process possible by measuring time between acoustic wave pulses sent from ships and echoes returning to the ships. Sonars used for echo soundings became far more sophisticated after WWII and have gotten increasingly high-tech in recent decades, but as early as the 1920s, European and North American ships were outfitted with echo sounders using the same scientific principles.
This is the point where our U.S. Hydrographic Office map enters the story. It’s a bathymetric map, the type of map used to denote shapes and depths underwater, and like most bathymetric maps it uses color and contour lines to distinguish between these various marine features. In between orange patches of land, the blue ocean appears, deeper and deeper blue at increasing depths, drawing our eyes to the royal blue point labeled “Milwaukee Depth.” While searching for more information about this map, I found a short article authored by a U.S. Hydrographic Office affiliate, entitled “The Deepest Sounding in the North Atlantic” from the March 1954 Proceedings of the Royal Society of London, Series A, Mathematical and Physical Sciences, Volume 222, Issue 1150. The article briefly describes the history of mapping the Puerto Rico Trench, and mentions the contributions of other ships whose names appear on the map where they measured nearby depths – the Dolphin and the Challenger – as well as the deep blue point in question, discovered only fifteen years before this article was written, named for the Milwaukee. (The map also labels a large part of the trench as the Brownson Deep, named for a commander with an earlier role in measuring the trench.)
According to the article, the depth recorded by the Milwaukee was confirmed by a later ship, the Gill, in 1952, which would have made this measurement current news in 1954 when this piece appeared. The agreed depth according to the article, after correcting the numbers that each ship calculated, was 4,770 fathoms, or 8,723 meters. (The “corrected” number initially indicated by the Milwaukee was 5,041 fathoms, which is the number visible on the map.) This calculation was made using relatively early sonar, and the article author stated that it should be confirmed using a wire line. Based on the timing and the fact that a very similar map to the one pictured above appears in the article, though in black and white, I believe that the 1939 U.S. Hydrographic Office map we have in our collection may be the very first published map to show the Milwaukee Deep, complete with the Milwaukee’s sonar measurements!
The suggestion to touch the bottom of the Milwaukee Deep in order to know its deepest point for sure was heeded in a different way, as described in Trethewey’s book: many decades later in 2019, a crewed submersible landed in the deepest part of the Milwaukee Deep and confirmed its measurement to be 4,580 fathoms, or 8,376 meters. In a new century’s continuation of what the U.S. Hydrographic Office was trying to accomplish in 1939, data from the 2019 dive is contributing to the Seabed 2030 Project’s mapping of the ocean floor.
Hopefully my metaphorical deep dive into the Puerto Rico Trench has piqued your interest in ocean mapping. GIS Day is currently sold out, but if you can’t make it or you missed your chance to sign up, be sure to check out the Event Videos page on our website in a few weeks, when the recording of Dr. Ferrini’s keynote talk will be posted! And, if the deepest point in the Atlantic isn’t exciting enough for you, you can read my colleague Julie Stoner’s blog post about mapping the Mariana Trench, Extremities of the Earth: The Lowest Natural Point.
To learn more about the Puerto Rico Trench and see some colorful three-dimensional renderings of it, check out the following resources:
- Lyman, J. U.S. Hydrographic Office. “The Deepest Sounding in the North Atlantic.” Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, Volume 222, Issue 1150.
- Cheadle, Michael. The Tectonic Setting and Geology of Puerto Rico and Its Surrounding Seafloor.
- Ten Brink, Uri. Chief Scientist U.S. Geological Survey. Puerto Rico Trench 2003: Implications for Plate Tectonics.
- National Geographic. Puerto Rico Trench.