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Civil War-era fortifications discovered under Alcatraz

Ground-penetrating radar uncovers remains of 19th-century fort beneath prison yard

Alcatraz Island.
Photo by Mathieu LE MAUFF/Shuttestock

Deep beneath the surface, Alcatraz has been keeping secrets. Now a team of researchers from Binghamton University, the Golden Gate Recreation Area, and the Concrete Preservation Institute in Chico have published the results of their experiments using ground-penetrating radar (GPR) to peak beneath the storied island and find out what the onetime prison is hiding.

The resulting paper, branded with the snappy title The Fate of the Historic Fortifications at Alcatraz Island Based On Terrestrial Laser Scans and GPR Interpretations From the Recreation Yard, appeared in the journal Near Surface Geophysics in January.

A team of eight went searching underneath the existing Alcatraz prison complex for signs of some of the older buildings once erected on the island during its days as an Army fort and military prison.

Since Alcatraz is a historic landmark, one can’t just go digging up the recreation yard for old bomb tunnels. Instead, the researchers employed “terrestrial laser scans and ground-penetrating radar” and compared the results to “historical documents, maps and photographs” to develop models of what appears to still be down there.

Here are some of their conclusions:

  • The history of fortifications on Alcatraz is longer and more substantial than most people realize: “Alcatraz is less known in its former military role as a 19th-century coastal fortification protecting the interests of a rapidly westward-expanding nation during the turbulent era of Manifest Destiny, the 1849 Gold Rush and the Civil War. The fortification, with its underground ammunition magazines and tunnels, is important from a military history perspective, marking the transition to earthen structures from the traditional brick and masonry constructions.”
  • But the design of the old Alcatraz fort was outdated before it was completed: “The heavy masonry structures erected to protect San Francisco Bay were obsolete even before construction had finished. During the Civil War, new rifled-cannon technology proved effective against Forts Sumter, Morgan and Pulaski.” The army shortly adopted a preference for earthen fortifications that stood up to new cannon technology better than the brick ramparts built at Alcatraz.
  • Future uses of the Alcatraz site smothered or destroyed historical remnants of past fortifications: “Construction of the military prison occurred between 1908 and 1912 and, at its completion, was reputed to be the largest concrete structure in the world. The grounds that would later become the recreation yard were originally used as a stockade. The stockade was built directly over two gun batteries. [...] These military elements, whether left intact or partly demolished, were in any case covered with construction.”
Photo by Ada Oliver
  • Radar scans verified the remains of 19th-century structures still underneath prison: “We hypothesize military features to exist beneath the recreation yard, as mentioned above, and these include the possible remains of traverse and bombproof tunnels. [...] In order to determine the location of these putative archaeological features, a GPR geophysical survey was performed at the recreation yard. [...Our] modeling and attribute analysis proved capable of detecting the precise location and extent of architectural and earthwork features.”
  • The researchers contend that similar technology can be used to uncover other buried archaeological assets without risking damage to the sites: “Archaeological excavations are inherently destructive and permanently remove buried artifacts from their primary context. [...] Many important historical structures excavations may not be desirable or even feasible. An ideal archaeological investigation of the subsurface based significantly upon an interpretation of geophysical data would be accurate and reliable enough to require minimal excavations, thereby preserving valuable non-renewable cultural resources in situ for future generations.”

The paper acknowledges that “drilling, trenching, and tunneling is certainly the most reliable method to assess [...] archaeological hypotheses,” but also stresses the potential applications of these newer and comparably non-invasive means.

Alcatraz proved an ideal place to field test the use of this technology since maps and records of buried sites still exist, which made it possible to compare radar images.

Authors T.S. de Smet et al. hope that, in the future, it will become more common to “figuratively rather than literally dig up an otherwise inaccessible but fascinating past” at other valuable but sensitive locations.

For a full look at their methods and findings, check out the full paper here.