I've been trying to avoid kidnappers, amoebas and any situation requiring me to speak Spanish. When I speak Spanish I always say things like, "When is the bathroom?" and "Please throw me that lusting taquito" and "I am in love with your dog."

The kidnapping is a constant source of conversation. To hear people talk, it's one of the steadiest job sectors in Mexico. The kidnappers may even have their own trade union at this point. You avoid kidnappers by not doing anything reckless, like hailing a street cab. The street cabs tend to be VW Beetles, and it's hard to imagine that someone in a VW Beetle (Herbie!) is a menace to society. But after hearing some harrowing stories, I would no sooner hail a cab than paraglide into the cone of Colima. (Colima is the volcano about 300 miles from here that's been erupting like crazy. "It's throwing lava bombs," a federal official told me. It's a miracle anyone gets out of this country alive.)

Microbes are harder to avoid. My main strategy is to stay away from water and, indeed, any substance that is even slightly moist. Fruit is out of the question. Also meat that's juicy. My host told me, "You can eat anything that's in a sealed bag, like crackers or corn chips." Meet the all-Frito diet.

In the shower I keep my eyes and mouth closed tight, lest a stray molecule of H2O serve as a vector for an amoeba with a nausea-inspiring name like flaggelopappomeceum. You want to avoid having a moment where you get a funny sensation in your plumbing and you think: "Uh-oh." Once you get the "uh-oh" feeling, it's only a matter of time before you start exploding like the guy in "Alien."

But not everyone feels the same way about water. The other day, while I was in the Zocalo, the big Spanish square at the center of town, I heard a racket coming around the corner. An orderly mob appeared, very middle-class looking, families with kids. The people were chanting something that I couldn't quite understand. I asked a cabdriver what they were saying. "Queremos agua," he said. And then I heard it clearly: "QUEREMOS AGUA!"

We want water.

The protesters came from a part of town that chronically lacks water. They want to wash their clothes. They want to bathe. They're thirsty.

I was standing on what used to be an island in the middle of a large lake, a basin surrounded by volcanoes. The Aztecs started building a city on the island in 1325. An engineer named Roberto Sanchez showed me one of their temples, the Temple of the Sun. We reached it by going to the rear of the massive Cathedral on the Zocalo, and then down a winding steel staircase and through a passageway so low we nearly had to crawl on our knees. We went on through gloomy chambers, until we saw it: the temple. It is a wall of volcanic blocks framing a manhole-sized carving of the radiant sun in faded pigments of red and yellow. The temple is under the church, buried by the Spanish, its remnants submerged in concrete and Catholicism.

The lake has long been drained away. The cathedral and much of the downtown is sinking into the mud. You can no longer see many of the surrounding volcanoes, because the smog is too thick. Mexico City has something like 18 million people, though there's no way to keep count with so many of them on the move. The human congestion is a wonder to behold: Imagine, if you will, Los Angeles without the elbow room.

The vast majority of the people arrived from the impoverished countryside in the last half-century. They drive cabs (legitimately), they work construction, they grind an organ and hope for a spare peso. You see people digging trenches through city streets with a shovel, because human muscle is so much cheaper than heavy machinery. The one power tool that seems to be everywhere is the jackhammer. They're ripping up the past, putting on a new layer.

A local professor told me that the city has stopped growing. The poor people don't come into Ciudad de Mexico anymore. They go, he said, to the United States.

When I look at Mexico City, I see the future of the planet: a world with too many people, too few resources, too many smog-choked megacities. Too many people in need of water. The United States can't put up a wall and hide from this future. With Mexico we share 2,000 miles of porous border. To paraphrase Thomas Jefferson in his first inaugural: We are all Americans; we are all Mexicans. Our destinies are intertwined.

Or, as I would tell people here in Spanish, "Now we eat the entire world." And with enough hand gestures, maybe they'll know what I mean.

Read Joel Achenbach weekdays at washingtonpost.com/achenblog.