The Submarine That Gave Its Crew A Mysterious Sickness

In 1863, German-American inventor and engineer Julius Hermann Kroehl founded the Pacific Pearl Company with the goal of profiting from pearl fishing in the Pacific. To streamline this labour-intensive process, he began constructing a submarine specifically designed to aid in oyster collection.

Traditionally, pearl diving relied entirely on human divers operating without breathing equipment. These divers descended to depths of 20 to 40 feet, gathering as many oyster shells as possible before surfacing—often holding their breath for up to a minute. Work typically occurred during low tide, which allowed access to deeper parts of the seabed. However, the diving window was short, lasting only two to three hours. Within this limited timeframe, divers made an exhausting 12 to 15 trips to the seabed. The pursuit of pearls and shells was not only gruelling but also perilous. Recognizing the risks and inefficiencies of this method, Kroehl sought to revolutionize the process, sparing his divers from unnecessary danger while improving yields.

A member of the Kuwaiti diving team searches for pearls during the annual Pearl Diving festival in Kuwait, in 2012. Photo credit: Raed Qutena

Submarines were still a novel technology with limited use in the mid-1800s, but their potential was undeniable. During the American Civil War, both the Confederacy and the Union used submarines for naval warfare, the most notable being the Hunley, which became the first submarine to successfully sink an enemy ship in combat. However, the Hunley sank shortly after the attack, likely due to the shockwave from its own torpedo. The Confederacy also worked on other submersible vessels, such as the Pioneer and the American Diver, though these efforts were largely unsuccessful due to technical challenges and resource limitations. The first submarine for the Union was the Alligator. It represented a significant technological leap with its compressed air system, air filtration capabilities, and innovative diver lock. This feature allowed divers to exit the submarine while underwater, enabling them to attach mines to enemy vessels—a precursor to modern underwater sabotage techniques.

Kroehl’s Sub Marine Explorer was 36 feet long and 10 feet in diameter, and weighed 80 tons. The vessel's sophisticated design featured three distinct sections— a compressed air chamber beneath the upper hull, a central working chamber for the crew, and a series of ten ballast tanks. To dive, a steam tender filled the high-pressure chamber with compressed air up to 200 psi. With the chamber charged, Kroehl and his crew opened seacocks from a central control station and flooded the tanks to make the vessel submerge. As the water rushed in, the air it displaced bled into a series of discharge pipes that fed through a central manifold and then exhausted out a valve in the conning tower. Kroehl trimmed the boat by regulating the flooding and with blasts of pressurized air to expel seawater from the ballast tanks.

Once at depth, pressurized air was released into the working chamber until the internal pressure had equalized with that of the ambient sea. Once this was done, the hatches in the bottom of the submarine could be opened without the water getting in. With the air within the working chamber completely holding back the water, the men slipped out of the hatches to collect and oysters and pearls.

For propulsion, the Explorer used a hand-cranked propeller capable of generating speeds of up to four knots. Additionally, the air within the vessel was refreshed by spraying seawater over chemical compounds, maintaining breathable conditions during prolonged dives. To surface, more of the pressurized air was blown into the ballast tanks to force the water out and make the craft buoyant.

The Sub Marine Explorer made several successful dives off the coast of Panama, demonstrating the potential of submarine technology for pearl diving. One particularly notable dive, reported by The New York Times, saw the crew recover an impressive 1,800 oysters—nearly 900 kilograms—in just four hours. However, the venture was not without its perils. After 11 days of intensive oyster hunting, all the men fell ill with what was vaguely described as a "fever," and some succumbed to the condition shortly thereafter. Local accounts suggested that these deaths followed after prolonged dives to depths exceeding 100 feet and then quickly resurfacing.

At the time, neither Julius Hermann Kroehl nor his crew understood the underlying cause of these tragedies— decompression sickness. Known colloquially as "the bends," this condition results from the physiological effects of breathing pressurized air underwater. As a diver descends, the increased ambient pressure forces gases such as nitrogen to dissolve into the blood and tissues. If the diver surfaces too quickly, the rapid decrease in pressure prevents the nitrogen from diffusing out gradually. Instead, it forms bubbles within the body, leading to a variety of symptoms.

The effects of decompression sickness can range from mild joint pain and rashes to more severe consequences like paralysis or even death. The nitrogen bubbles often collect in major joints, such as the knees or elbows, causing intense pain that forces victims to bend over in agony—a hallmark of the condition and the origin of its nickname. Tragically, Kroehl and his crew were unaware of the need for controlled, gradual ascents to avoid this life-threatening complication.

The connection between reduced ambient pressure and the formation of air bubbles in living tissue was first demonstrated in 1670 by Robert Boyle. Using an air pump, Boyle conducted experiments on birds and vipers, initially observing that they died from asphyxiation. However, later experiments revealed symptoms resembling what would eventually be recognized as decompression sickness.

“An Experiment on a Bird in the Air Pump”, a painting by Joseph Wright of Derby, 1768, depicts Robert Boyle’s famous experiment.

The first documented case of decompression sickness in humans came in 1841 when French geologist Jacques Triger observed the condition in workers using his pressurized caisson method to dig through waterlogged ground. The ailment became known as “caisson disease,” as it commonly afflicted laborers working in pressurized caissons during large-scale excavation and construction projects.

Caisson disease proved to be a major hazard in engineering feats of the 19th century. During the construction of the Eads Bridge in St. Louis in the 1870s, 15 workers succumbed to the mysterious illness. The condition also played a prominent role in the building of the Brooklyn Bridge, where project leader Washington Roebling suffered debilitating symptoms after spending time in the pressurized caissons, forcing him to supervise the remainder of the construction from his home.

Elsewhere in New York, during the construction of the Hudson River Tunnel in 1889, decompression sickness claimed multiple lives. Contractor's agent Ernest William Moir noted the recurring deaths among workers, confirming the severe risks posed by prolonged exposure to pressurized environments and rapid decompression.

By the late 19th century, the scientific understanding of decompression sickness had advanced significantly. Once it was recognized that the symptoms were caused by gas bubbles, and that recompression could relieve the symptoms, further work showed that it was possible to avoid the debilitating effects of the condition by slow decompression. British physician John Scott Haldane’s research into decompression produced the first recognised decompression table in 1908, providing precise guidance on ascent rates and stop durations based on depth and time spent underwater. These tables were a milestone in diving safety and were swiftly adopted by the Royal Navy, setting a global standard for safe diving practices.

A modern diving table.

Julius Hermann Kroehl died in September 1867, officially of “fever,” though there remains speculation that his death resulted from decompression sickness acquired during experimental dives with the Sub Marine Explorer. His widow disputed this theory, attributing his death to malaria contracted during his military service in the Vicksburg campaign, supported by accounts from witnesses and medical statements.

Without Kroehl’s leadership or the financial resources to sustain operations, the Pacific Pearl Company abandoned the venture. The crew returned to New York, leaving the Sub Marine Explorer beached and idle. In 1869, a new engineer and crew resurrected the craft, bringing it back to the Pearl Islands to continue harvesting oyster shells and pearls. However, deep dives proved fatal for many workers, who fell ill or died. The repeated tragedies forced the crew to abandon the submarine in a cove on San Telmo Island, where it eventually sank into obscurity.

For decades, the rusting hull of the submarine remained a curiosity among locals, who believed it to be a relic of World War II. In the early 2000s, James P. Delgado, an archaeologist with the U.S.-based Institute of Nautical Archaeology, investigated the site after hearing claims that the mysterious vessel was a Japanese two-man midget submarine, supposedly left behind following an aborted attack on the Panama Canal. What Delgado discovered, however, was far more extraordinary—a pioneering piece of 19th-century engineering that predated modern submarine designs by decades.

Despite renewed interest in the Sub Marine Explorer, efforts to study and potentially recover the vessel have been hampered by logistical and financial challenges. As of 2010, the submarine remained submerged, its metal hull severely corroded from decades of exposure to saltwater. Analyses confirmed that the craft had reached a critical stage of deterioration, and without immediate preservation measures, it faced irreversible decay and eventual loss.