A thousand years ago, the region around Mexico City bore little resemblance to what you’d see there today. For one thing, it was largely underwater. A system of lakes stretched across what’s now called the Basin of Mexico. People lived on artificially constructed islands within the lakes. They farmed corn, beans, squash, and other crops on raised fields that they mounded up in the swampy shallows.

The Basin of Mexico had more water long ago, but it had fewer large animals. There were no cattle or other big, domesticated creatures in the Americas to provide meat or milk. This has led researchers to wonder how ancient populations got enough calories and protein. In the 1970s, “Some archaeologists even argued that this explains cannibalism among the Aztecs,” says Associate Professor of Anthropology Kristin De Lucia. But researchers now say Aztec cannibalism was infrequent and ritualistic — far from a dietary staple.

It seems intuitive that the people who lived there long ago must have taken advantage of the lakes’ flora and fauna for food and other needs. But just as the lakes have vanished — drained over hundreds of years to make way for new development — the evidence of those ancient practices is hard to see now. “A lot of the resources that people exploited from the lake are actually really hard to identify archaeologically because they don’t preserve,” De Lucia says. Unlike pottery and stone tools that can survive for centuries or millennia, items such as fishing nets are long gone.

De Lucia has been looking for subtler hints of how the ancients used Mexico’s lakes at a place called Xaltocan (pronounced hall-toe-CAHN). Today it’s the site of a small town, but buried under the houses and yards are artifacts from the people who lived there in past centuries. De Lucia focuses on a period from about 900 through 1250 AD. Back then, Xaltocan was one of the lake system’s artificial islands. In a recent paper, De Lucia argues that Xaltocan holds plenty of evidence about how its inhabitants used the lake’s resources—if you know where to look.

One way De Lucia has tracked down nearly invisible evidence is by thinking small. In the compacted dirt floors of ancient homes, she looks for bits of embedded debris that were too tiny to sweep away. She sifts these so-called microartifacts out of the dirt while she’s in the field.

Then, back at her Colgate lab, students use microscopes or magnifying lamps to help sort through the minuscule objects: a crumb of pottery, a flake of obsidian, a fish scale that someone stepped on and ground into the floor. They “painstakingly” count and weigh different types of microartifacts, De Lucia says. Then she uses the data to create a kind of map.

In the hunt for evidence of how Xaltocan’s residents used the lake, “We didn’t just look at what was left behind, but also the spatial organization of the artifacts,” De Lucia says. “The first time I did this I thought, oh my God, this is just never going to show anything.” But when she plotted the data in space, patterns emerged.

For example, although fish bones have been hard to come by at Xaltocan, sifting through the dirt floors has turned up lots of fish scales. The scales are most abundant in certain parts of certain rooms. De Lucia has also done chemical analysis of the soil, and found that areas with many fish scales also have high levels of sodium. This likely came from salt that residents used to dry and preserve fish, De Lucia says.

The fish scales and salt also overlap with larger artifacts that fit the theme, De Lucia says. For example, she’s found a forked needle made of bone that was likely used to sew fishing nets, and little ceramic circles that are probably weights for holding the nets down.

Other clues suggest ways besides fishing that Xaltocan’s residents made use of the lake. For example, De Lucia has found plentiful bones from ducks that seem to have been processed and cooked. Remains of turtles have popped up too, along with the occasional frog or toad. She’s also found stones that residents probably used to hammer the lake’s reeds and weave them into mats or baskets, although the reeds themselves disappeared long ago.

Insects and algae from the lake would have been handy protein sources for the residents too, De Lucia says. “I have no doubt that they would have used them.” But these ephemeral objects didn’t leave any archaeological trace that she’s been able to find yet.

She may be running out of time. Mexico City’s new airport is under construction near Xaltocan. “From what I’m seeing from photographs, the town is basically covered in clouds of dust from the construction,” De Lucia says. She and her colleagues are concerned that some archaeology is disappearing for good. “The building where we had stored all of our artifacts was torn down,” she says. “So we don’t even know where anything is or what is going on.”

For the past two summer excavation seasons, the pandemic has prevented De Lucia from visiting Xaltocan. “We were powerless to do anything,” she says.

Even if future excavations are impossible, though, “There’s a lot more work to be done with the materials that we do have,” De Lucia says. She hopes to gain not just a clearer view of how ancient people lived, but lessons about how we in the present day can live better.

“Raised-field agriculture was one of the most highly productive forms of agriculture that humans ever invented,” De Lucia says. Out of the mucky edges of lakes, people created fertile farms. And without the resource-hungry cattle or pigs that we have today, they thrived on the lake’s fish, fowl, and other resources.

“They really utilized the environment in sustainable and efficient ways,” De Lucia says. “I think we can learn a lot from studying the ways people did things in the past, because they knew their environments very well.”