A Case for Circumstantial Evidence

Long before satellites and electronic navigation, Pacific Islanders learned to recognize distant islands through changes in ocean swells, clouds, and wildlife. An island could influence its surroundings well beyond the distance from which its land could be seen.

Modern science uses the same principle in remarkably different ways. Researchers can identify animals from traces left in water, investigate underground structures by observing the ground above them, and even detect ships using the movements of seabirds.

What the water remembers

In 2018, an international research team collected 250 water samples from Scotland’s Loch Ness, searching for genetic traces of the animals living there.

The investigation attracted attention for an obvious reason: the possibility of finding evidence of the legendary Loch Ness monster.

Animals continually shed genetic material through skin cells, waste, and other biological processes. This environmental DNA, or eDNA, can remain in water after an animal has moved elsewhere. Researchers collect samples and compare the recovered sequences with those of known organisms.

The Loch Ness investigation found abundant eel DNA, but no evidence supporting the theory that a prehistoric marine reptile inhabited the lake. Researchers could not rule out unusually large eels as an explanation for some sightings.

The same technique has more practical applications. Scientists have recovered DNA from seawater left behind by passing killer whales, demonstrating that genetic evidence can be collected without capturing or directly examining the animals.

Watching the ground move

In California, scientists use satellite radar to measure changes in the land surface caused by groundwater pumping.

As groundwater is withdrawn, changes in underground pressure can cause sediment layers to compress, lowering the surface above them. In some locations, the ground partially rebounds when water returns. Elsewhere, compaction is permanent, reducing the aquifer’s future storage capacity.

Using interferometric synthetic aperture radar, researchers compare observations made at different times to detect small changes in ground elevation.

The movement of the land helps reveal what is happening to water stored far beneath it.

Finding ships with seabirds

Researchers investigating illegal fishing have found an unusual surveillance partner: the albatross.

These seabirds travel enormous distances across remote oceans, often encountering fishing vessels far beyond the reach of ordinary patrols.

Scientists have equipped albatrosses with small devices that detect radar emissions from nearby ships. When a bird encounters a vessel, its equipment can record the signal and location.

Researchers can compare those detections with conventional vessel-tracking records, potentially identifying ships operating without broadcasting their positions.

An untracked vessel is not necessarily engaged in illegal fishing. But the technique offers a way to discover activity in enormous areas of ocean that would otherwise be difficult to monitor.

Looking through a volcano

Another unusual detection method takes advantage of particles constantly arriving from space.

Cosmic rays interacting with Earth’s atmosphere produce muons, subatomic particles capable of passing through substantial amounts of rock.

Dense material absorbs more of these particles than less-dense material. By measuring the muons that emerge from different directions, researchers can estimate the density of material inside a mountain.

The technique, called muography, has been used to study the interiors of volcanoes. Researchers in Japan have even reported observing magma moving up and down within a volcanic conduit.

The mountain remains opaque to ordinary observation, but the particles passing through it carry information about what lies inside.

When plants become instruments

Indirect detection sometimes relies on something considerably more familiar than satellites or subatomic particles: vegetation.

A leaking underground water pipe can create a patch of unusually moist soil. Plants above the leak may remain healthier during dry periods, or their temperature and infrared reflectance may differ from neighboring vegetation.

Researchers have investigated using those differences to identify suspected underground leaks through remote sensing.

This relationship between plants and buried objects also provides one of archaeology’s most remarkable discovery methods.

The discoveries that appear during drought

A field of crops may conceal the remains of a settlement, defensive ditch, or building that disappeared hundreds or thousands of years ago.

Those structures can change the properties of the soil above them.

A buried stone wall may restrict the depth available to roots, while an ancient ditch filled with loose soil may retain additional moisture. During normal growing conditions, the differences can be difficult to distinguish.

During drought, they may become striking.

Crops above buried walls can dry out sooner, while those growing over ancient ditches remain green longer. From the air, these patterns can reveal the outlines of buildings, roads, enclosures, and entire settlements otherwise invisible at ground level.

Archaeologists call these patterns cropmarks.

The buried structures have not changed. The drought has changed how strongly their effects appear in the plants above them.

A field surveyed repeatedly under ordinary conditions might reveal an ancient settlement during an unusually dry summer. The camera has not become more sensitive, and the photographer may be looking at exactly the same location. The environment has made the difference visible.