Fresh insights into a secret group of ocean scientists who helped save American troops and submarines in World War II


Lethal Tides: Mary Sears and the Marine Scientists Who Helped Win World War II

Catherine Musemeche

William Morrow, New York

Hardcover, eBook, audio book, 978-0-06-299169-0, 320 pages

August 23, 2022


Austin writer Catherine Musemeche’s informative, well-written new book offers fresh insights into a secret group of ocean scientists who helped save American troops and submarines in World War II.

Lethal Tides tells the story of a small office attached to Naval Intelligence in Washington, D.C., and its leader, Mary Sears, who in 1943 became the U.S. Navy’s first oceanographer. Incredibly, nearly midway into the 20th century, the Navy still knew little about the oceans through which it steamed, and it frequently showed little interest in new ideas gleaned from ocean research, such as better ways to remove speed-slowing barnacles from ships’ hulls.

Discrimination against women in the workplace remained strong, even in wartime. It took Sears, a Naval Reserve officer, more than a year to convince attack planners to pay closer attention to the oceanographic data that she and her small staff, mostly women, were turning into intelligence reports. They provided important details about tides, barrier reefs, currents, coastal water depths, and other measurements related to islands and coastlines held by the Japanese and Germans. Using existing data plus personal research, Sears also demonstrated that submarines often could avoid or confuse enemy sonar by diving beneath thermoclines, areas where warm water met cold water and temperatures dropped quickly with depth. 

As the author notes, some of America’s initial amphibious landings under fire in World War II’s Pacific campaign had been costly achievements. Many Marines, soldiers, and sailors had been killed or injured before reaching shore—and not always by enemy fire. Some landing craft and amphibious vehicles had run aground on reefs or gotten stuck in shallow water and become easy targets. Others had been swamped or overturned by high waves or thrown onto beaches and destroyed.  Meanwhile, some troops had been forced to jump from damaged vessels into water deeper than they expected, and they drowned, pulled down by their weapons, ammunition, and equipment.

The planners of these first amphibious assaults on Japanese-held islands had focused mainly on gun emplacements and defense preparations. They had given scant attention to tide levels and sea-bottom conditions near landing beaches. The high casualties, plus the persistence and expertise of Mary Sears and her group, finally brought about new thinking in the Pentagon. When U.S. forces went ashore on Okinawa in April 1945, the author notes: “As Sears predicted, the waves on the westerly shore were calm. By 1600 hours [4 p.m.], fifty thousand troops were ashore. This was the opus of the amphibious campaign, an orchestra tuned to perfection performing at its best. This was the kind of landing that naval commanders had envisioned from the start of the Pacific Campaign and strived to roll out.”

In Lethal Tides, Catherine Musemeche skillfully blends biography, World War II history, personal interviews, and other materials from numerous sources, including Fredericksburg’s National Museum of the Pacific War. When Japan attacked Pearl Harbor, Mary Sears was a zoologist with a Ph.D., studying plankton off Peru’s coast. She returned to the U.S. and tried to join the Women’s Naval Reserve but didn’t pass the physical. She returned to work at Woods Hole Oceanographic Institute, where she was a research assistant. A series of events soon led to her being singled out for a job in the Navy’s Hydrographic Office. The work, however, required a high-security clearance and being in the military. This time, Sears received a medical waiver and was sent to eight weeks of Naval Reserve officer training. 

Once Sears became the Navy’s first oceanographer, expectations initially were low. The author explains: “She had never been asked to set agendas, call meetings, or give people orders, much less make sure they carried them out, but she was going to have to do those things to get the military the information they needed to win the war. She was going to have to take the lead.” Starting with one desk, she “transformed naval oceanography into a fifteen-person unit,” and the admirals and generals of the Joint Chiefs of Staff finally began calling on Lt. (jg) Mary Sears “to furnish critically valuable information for use in combat operations.”

Lethal Tides shines an inspiring spotlight on a little-known chapter of American military history and on how a well-qualified female scientist and her staff overcame obstacles and made important contributions that helped win the war in the Pacific.     

Catherine “Kate” Musemeche trained at one of the elite children’s hospitals in the country, Children’s Memorial Hospital of Northwestern University in Chicago and has been a pediatric surgeon for more than three decades. Musemeche also has an MBA from the Anderson School of Management at the University of New Mexico and a JD from the University of Texas School of Law. Musemeche’s first book, Small, was longlisted for the E.O. Wilson/Pen American Literary Science Award and was awarded the Texas Writer’s League Discovery Prize for Nonfiction in 2015. She has also contributed to the New York Times’ “Motherlode” blog, and EMS World. She lives in Austin, Texas.