Generally, archaeologists search for subtle indicators of buried sites, such as potsherds peeking out of the soil or grassy mounds in otherwise flat terrain. To survey regions for these clues, they use images or data collected from above — by satellites and aircraft — and below the surface, using methods like ground-penetrating radar.

Or, researchers do good old-fashioned footwork. In what's known as a systematic survey, they walk a landscape, in orderly paths, looking for surfaced artifacts and other hints of underground sites. Researchers plot finds with GPS to produce maps, revealing areas with lots of artifacts — a good clue for where to dig.

Surveys may cover a small region, but thoroughly. For example, researchers spent five years in the 1990s scouring an area of just 2.5 square miles on the Mediterranean island of Cyprus, recording artifacts from the past 5,000 years. By the study's end, they counted 87,600 potsherds, 3,092 stone artifacts and 142 promising dig sites.

This kind of ground reconnaissance is relatively low-tech, but effective. Plus, it’s great exercise. The challenge is knowing what to look for. It takes experience to distinguish, say, between an ordinary rock and a stone tool, or between a natural hill and grass-covered ruins.

If walking back and forth for days (or years) sounds tedious, you could also try the aerial approach. Using space and airborne remote sensing, archaeologists often identify archaeological features using datasets collected by satellites, planes and drones.

A bird’s-eye view makes it easier to spot certain landscape features caused by buried remains. For example, plants will grow thickly above buried wooden structures, and more sparsely above stone. Comparing these to regular vegetation can be a hint of something below.

But the contrast may only be discernible from above, and becomes starker during dry spells. Amidst a 2018 drought, for example, a drone flying over wheat fields in Ireland captured the outline of a roughly 5,000-year-old monument, similar to Stonehenge but made of timber. The circular image in the crops had never been seen before, and disappeared after the drought was over.

In arid regions like the Middle East, satellite images offer great aerial shots. Even Google Earth has proven effective for finding unknown sites in Afghanistan, Saudi Arabia and elsewhere. Beyond discovering new sites, aerial views help archaeologists study regional connections between sites, such as Bronze Age roadways in Syria identified from satellite shots. In this case, the study used older satellite photos — snapped during 1960s Cold War spying. The declassified CORONA satellite shots were better than present-day views because recent urbanization and farmland had covered undeveloped land, erasing ancient features still visible a few decades ago.

Unfortunately, in densely forested areas, satellite photos will only show treetops. In these regions, archaeologists have benefited from LiDAR, or Light Detecting and Ranging, one of the newest tools in remote sensing. Flown over a landscape, the instrument sends nondestructive laser pulses to the ground and measures the reflected beams. The data creates a detailed 3D map of terrain, with vegetation digitally removed. The ability to see through trees in this way has led to tremendous discoveries. For instance, a team using LiDAR in Guatemala recently found 60,000 structures from the ancient Maya civilization — hidden, until now, beneath thick foliage.

After archaeologists identify a promising area, they need to select a specific spot to excavate. For this, they usually dig test pits or take cores — small probes into the ground to determine what’s really down there.

There are also a number of ways to peek underground digitally without digging, including ground-penetrating radar (GPR) and electrical resistivity. The latter works for the same reason that plants grow more or less lushly over different materials. Underground features like a grave or wooden foundations will make soil damper, so it will conduct electricity more easily than dry, stony ground. For these geophysical methods, researchers drag instruments on little trolleys across the area of interest. The devices collect data on subsurface anomalies — suggesting perhaps a wall, structure or burial.

Despite the efforts of archaeologists, many sites are discovered accidentally by non-archaeologists — including some of the most famous. Farmers digging a well unearthed the 2,000-year-old terracotta army of China’s first emperor. Utility workers found Templo Mayor, the Great Temple of the Aztec Empire, right beneath the streets of Mexico City. Construction projects often run into ancient remains, and most countries have laws to ensure archaeologists are called in when that happens.

You never know what lies below. But before they dig, archaeologists usually have a good idea.