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On June 6, 1944, thousands of men rained down from the skies onto the battlegrounds of Normandy. After five grueling years of war that shook the globe, D-Day’s victory swept the Allied nations into a wave of celebration.

Generations later, D-Day is still commemorated, honoring the brave soldiers who shaped history.

LaRue ‘Tex’ Wells, a retired U.S. Geological Survey fisheries scientist, watched D-Day unfold from a bird’s eye view as a pilot dropping paratroopers over the battlefield. While Tex was flying through the skies, a device known as a bathythermograph, or BT, was plummeting beneath the sea surface, monitoring temperature and depth to keep U.S. Navy submarines out of enemy sonar range. Nine years later, on a research vessel on Lake Superior, Tex used the very same bathythermograph technology to conduct scientific research.

The Invaluable Device

Because sonar only works in a specific range of temperature and pressure, Navy submarines could use the BT data to strategically position themselves in a pocket of water that masked them from the enemy. This ability to distinguish safe zones from dangerous parts of the water column became an instrumental part of submarine warfare.

The Bathythermograph
The mechanical bathythermograph was invented as a tool to understand the temperature layers of the ocean. The torpedo-shaped device was attached to a long cord and dropped off the side of a boat. As it sunk towards the sea floor, an internal needle scratched a line on a piece of glass that represented temperature relative to water depth.(Credit: Tara Bell, USGS. Public domain.)

Almost overnight, BTs were in high demand and were rapidly manufactured to meet wartime needs. To keep pace with the heavy stream of data that was constantly coming in, the University of California, Division of War Research and the Woods Hole Oceanographic Institution initiated large-scale data processing efforts. The processing involved independent calibration of each BT slide and then timely transmission of the information back to the Naval officers to use. These massive efforts resulted in an influx of BT records that at the time were used for pinpoint observations, but now provide a chronology of ocean temperature patterns.

While the applications of BT data have shifted over time, the information continues to hold value in society. Tex, a man who experienced the various phases of the BT, can shed some light on how the transition occurred.

Although Tex and the BT both played significant roles in the war, Tex said he never saw a BT until he was hired by the U.S. Bureau of Commercial Fisheries in Marquette, Michigan, studying fish populations in Lake Superior on the research vessel Cisco. When he began his career in 1954, collecting BT data was simply routine and good practice on a ship, not necessarily for research purposes.

The Research Vessel Cisco
The Cisco was a vessel used for research on Lake Superior in the mid-1900s.(Credit: U.S. Fish and Wildlife Service, USGS. Public domain.)

“Originally collecting bathythermograph data wasn’t part of any study—it was just something we did,” said Tex. “Between each port we would take a BT reading every 30 minutes.”

This phase of post-wartime data collection created the continuous timeline of BT data that scientists now examine in a new light.

Starting in 1954, Tex and others collected BT data from thousands of points across the Great Lakes, resulting in a complete temperature record for the region. Having so many data points gives scientists an opportunity to connect the dots and recognize any significant trends in water temperature. Researchers can use these data to improve their understanding of the relationship between temperature patterns and ecosystem dynamics.

“Great Lakes science and management are increasingly recognizing the role that climate plays in driving food web and population dynamics,” said USGS Research Fishery Biologist Dr. Brian Weidel, who studies Lake Ontario prey fish and uses the historic temperature data to understand how Lake Ontario water temperature changes have influenced the fish community.      

Preserving the Legacy

From glass slide to photograph
Great Lakes Science Center’s bathythermograph collection exists in an inaccessible form. The original glass slides (top) are recovered and scanned, leaving scientists with black and white images (bottom). The temperature curves on these scanned images must be interpreted digitally.(Credit: USGS, U.S. Geological Survey. Public domain.)

In order to reap the benefits of this wealth of BT information that exists, these historic data must be recovered and preserved. The USGS Fort Collins Science Center’s Data at Risk (DaR) project recently preserved and released “Bathythermograph Data, Lake Michigan, 1954” (, a subset of the extensive collection of BT data stored at the USGS Great Lakes Science Center.

Historic data (also called Legacy data) often require transformation from their original, inaccessible format to a form that is compatible with modern systems. In the case of Lake Michigan data, each BT reading was translated from its original glass slide form to a digital format using Web Plot Digitizing software. Format transformation is a vital component of the preservation process that revives the value of the information; without accessibility, the decades of data remain out of reach.

LaRue "Tex" Wells and Scott Nelson hold a bathythermograph
Retired and current USGS scientists from the Great Lakes Science Center display a bathythermograph. The device was invented in the early 1900s and collected temperature information in the Great Lakes up until the 1990s.(Credit: Jean Adams, USGS. Public domain.)

Technology Keeping Pace with Society

The BT is a prime example of how both technology and the value of data can change over time. The winds have changed since Tex was flying through war-torn skies; the concerns of society have shifted to an extent from submarine warfare to the wellbeing of our landscapes. The USGS has used the BT to keep pace with the changing world.

Advancements such as the electronic water profiler have enabled the continued collection of temperature at depth data through modern technological times. It is thanks to the versatility of technology and the protocols implemented on ships that the long-term record of information exists and now has the potential to be recovered and reused by the USGS.

Historic environmental information is invaluable in extending the scientific record and assigning it new purpose. With a modern perspective and innovative technologies, the temperature points that were once used to dodge enemy submarines and record cruise data can now be realigned to examine the patterns of our changing ecosystems. The BT serves as an example of how the USGS and the scientific community can continue to learn from information acquired decades ago. By using modern technologies to preserve and release historic information into the community, society can continue to benefit from the investment in historical data that can never be recollected.

Story written by Cristiana Falvo.

Water profiler
A modern electronic water profiler is used aboard the 104’ USGS research vessel Sturgeon on Lake Huron in April 2017 to perform the same function as the mechanical bathythermograph in the mid-20th Century.(Public domain.)

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