Here’s what you’ll learn when you read this story:

• Archaeologists recently discovered a Roman canal from roughly the 1st century C.E. hidden beneath German farmland.
• The navigable shipping channel connected Roman boat traffic on the busy Rhine River to a Roman “fortlet” and nearby settlements.
• The highly engineered canal restructured the landscape of Germany’s Upper Rhine Graben.

Archaeologists working in German fields detected something odd below the surface: an unusually straight, riverbed-like anomaly that, upon further investigation, they discovered was actually an early Roman canal that once connected the Rhine River with Roman Empire “fortlets” and settlements. The canal’s central purpose appears to have been related to the Roman fortlet at Trebur-Astheim, in the Hessische Ried region of Germany’s Upper Rhine Graben.

The canal was a major engineering project that reshaped the environment as early as the first century C.E., and it offers one of the only examples of a navigable Roman canal constructed north of the Alps in the early Middle Ages. In a new study published in the journal Land, a team of experts employed a number of methods, including magnetic gradiometry, electrical resistivity tomography, and sediment coring, to investigate the site and learn how much of this side channel was from deliberate human construction. The answer? All of it.

“The canal at the burgus at Trebur-Astheim was not a secondary channel of the River Rhine but rather an artificial canal belonging to the Landgraben/Schwarzbach fluvial system,” the authors wrote. “We suggest that the discovered canal at Trebur-Astheim was constructed during a time when the rest of the River Landgraben was intensely used by the Romans.”

Experts uncovered a channel that was once eight feet deep and 50 feet wide, critical in water management in the region to “guarantee the transportation of material and troops, securing the territory of the Roman Empire.”

Constructed sometime between the first and fourth centuries C.E., the canal offered a direct link from the Rhine to the burgus at Trebur-Astheim, a late Roman fortlet used as an inland harbor, “providing a secure docking site for the transport of ware and troops along the Roman-Germanic border.” It’s one of at least five such fortlets along Rhine tributaries, this one built under Emperor Valentinian I between 364 and 375 C.E. It features a main building with protective ditches and walls leading toward an artificial basin.

“The burgus at Trebur-Astheim was built to provide a secure inland harbor on the eastern fringe of the late Roman territory,” the authors wrote. The multi-story structure created a fortified perimeter and connected to the waterway in a style consistent with other fortifications constructed during the reign of Valentinian I.

The now-buried canal offered the Romans a water-based superhighway through a region they had a firm grasp on. The route even ran through the site of an earlier Roman military camp that had been occupied briefly around 14–20 C.E.

Sediment cores revealed a clear sequence of Paleozoic bedrock covered by Pleistocene river gravel and topped by alternating layers of sandy channel deposits and fine-grained silt sediment. Experts believe the canal functioned into the eighth century C.E., well after the fall of Rome, as Merovingian and early Carolingian communities maintained the passage. It may have even been critical in helping build the royal palace at Trebur, which made its first appearance in historical records around 829 C.E. When dredging ceased, the canal filled in, and it was eventually subsumed by the farmland the archaeologists were searching.