American Scientist

We might pass graves in cemeteries so often they become invisible to us. There are other graves that are invisible intentionally, to literally cover up a nefarious deed. Clandestine grave sites—such as those of “The Disappeared,” 17 victims who the Irish Republican Army admitted to abducting and killing, after their corpses were hidden for decades—are notoriously difficult to find. Intelligence work, along with a variety of scientific techniques, returned many of “The Disappeared” to their loved ones. This huge task required acres and acres of land to be searched. What is the best way to go about such a job? It is a question that challenges law enforcement and forensic scientists the world over, and science is constantly adding new potential methods to explore.

As an analytical chemist doing research in the area of forensic science, I find the identification of clandestine grave sites a fascinating area of research and development, often frustrating in practice and tinged with an array of emotions. Imagine having to search thousands of square kilometers of an entire country for mass graves related to war crimes or a national park for a serial killer’s “dump site.”

Scaling way down to a quarter acre backyard decreases the search area, but not the number of search questions. Big site or small, we can’t use the technique from the book Holes of just-make-everyone-keep-digging. Equipment, terrain, climate, along with available workforce, workable hours, and budget all impact searches—no matter their size. Beyond these issues, criminal or historical excavations require the utmost care and caution, proceeding sometimes at a snail’s pace—or slower even! Along the way, we usually have to document—in writing and images—each layer, item, and observation. It’s a big job, whether the search area is a backyard or park.

To meet the challenges of finding clandestine grave sites, a diverse set of tools are utilized, with the development of new strategies an ongoing area of research. Classic techniques are still being used—sometimes with a twist. For example, aerial photography, used for several decades and now employing drones, allows investigators to identify areas of disturbance to vegetation and soil within large areas. Such disturbances can also change soil properties such as magnetism (magnetometry) and the flow of electrical current (resistivity and electromagnetic conductivity). Perhaps the most famous technique is another that uses electromagnetic energy—ground penetrating radar.

Instruments for ground penetrating radar deliver electromagnetic energy in the form of radio waves to the ground via antenna. When radio waves hit areas of soil discontinuity, some portion of the wave is reflected back and detected by a signal receiver antenna. Properties of the reflected wave are correlated to soil properties and the depth of soil discontinuity. GPR, along with the previously mentioned geophysical techniques, can provide focus to searches—a sort of indication to look at a spot more closely. These techniques do not reveal bodies hidden under layers of soil or vegetation—no matter what shows like CSI say. These techniques help triage areas to help prioritize which spots are worth a second look and perhaps even excavation.

The noninvasive nature of geophysical techniques makes them invaluable tools. They allow experts to study what lies beneath without having to actually go beneath, saving excavation time and expense, along with preserving crime scenes or historical sites. As eluded to above, the identification of soil discontinuity, while valuable, does not equate to “corpse was buried here.” To get us closer to that type of discrimination, a variety of other tools are used, including human remains detection (HRD) dogs.

HRD (“cadaver”) dogs are specially trained canines, using their great sense of smell to sniff out and alert to scents of human decomposition. As with geophysical techniques, HRD dogs can help triage search areas and, when used together, greatly assist investigators on deciding where to dig. But, like geophysical techniques, HRD dogs have their limitations. For instance, they may alert to a corpse—just not a human one. Extensive training does help minimize “wrong” alerts, when dogs are exclusively trained with human remains and their relevant target odors.

Target odors are an active area of research—and not just because of their use in training HRD dogs. The host of volatile organic compounds (VOCs) responsible for that very distinct smell of death, i.e. “cadaveric VOCs,” can help forensic scientists estimate the postmortem interval (time since death) and develop sensing techniques to pinpoint gravesites. To assist in identifying clandestine graves via chemical sensors, the Decompositional Odor Analysis (DOA) Database was created by Oak Ridge National Laboratory (ORNL) and University of Tennessee’s Decay Research Facility. ORNL used DOA to develop LABRADOR, which is short for “light-weight analyser for buried remains and decomposition odor recognition.” This device’s canine name is intentional. LABRADOR falls into the “electronic nose” class of analytical devices designed to mimic the olfactory systems of either us or our dog friends.

Beyond sniffing out death, either using real or electronic noses, the chemical changes that mark a decomposing cadaver can help identify gravesites. The release of nitrogen-containing organic compounds, such as amino acids, into soil via cadaver decomposition has been exploited by researchers to presumptively detect grave soil using classic ninhydrin chemistry and an established soil assay. This chemical analysis does require direct sampling of soil, providing only a presumptive (“possible here”) test. Why only presumptive? A decomposing human body isn’t the only source of nitrogen-containing organic compounds. Like other presumptive tests, it is another tool that can assist in triaging areas of interest.

Cadaver decomposition releases a lot of chemicals and attracts all sorts of living things. Forensic entomology, made nearly a household name by forensic TV shows and books, can enable experts to narrow-in on potential gravesites by the appearance of specific insects in a given location. Insects and dogs aren’t the only living creatures that can alert us to gravesites. Soil and human-associated microbes could potentially be HRD single-cells, perhaps even providing post-mortem interval information.

New methods are being tested, one even putting single-celled organisms to work. In a recent PLoS ONE paper, researchers Kelly Cobaugh, Sean Schaeffer, and Jennifer DeBruyn studied the who, what, and when of soil microbial communities below decomposing human cadavers, while also looking at the microbes released from decomposing human cadavers. As the researchers state, we know “microbes are responsible for recycling carcass-derived organic matter.” But not much is known about exactly which microbe taxa are involved, when, and for how long. This small study monitored four buried cadavers and indicated that soil microbial communities can be differentiated into periods of “Active Decay” (the period of most rapid mass loss) and “Advanced Decay.”

When these researchers began their work, they assumed “members of the human microbiome would be introduced to the soil environment, contributing to the altered structure, but would not persist outside their natural environment.” I assumed the same. Both the authors and I were in for a surprise!

Their data indicate “that human-associated microbes persist in the soils for surprisingly long periods of time, suggesting a possible role in decomposition.” These members of the human microbiome have potential for clandestine grave identification, because, as researchers wrote, “Their low abundances in natural soils and significant increases during decay render them good candidates as biomarkers.” While the authors see them as markers for estimating postmortem interval, I wonder if soil microbe analysis could—with more research and method development—become another tool in our clandestine grave site detection toolkit.

A toolkit, not a single tool, is what a job like clandestine grave detection requires. The diversity of burial time, climate, terrain, analytical resources, personnel, and other factors means we need a variety of tools based on a number scientific disciplines, including microbiology, entomology, chemistry, physics, and geology.