Carlos Poo's house is located on an unpaved street that is not on the map. You reach it by driving past mounds of garbage where the poor scavenge for tin cans but also--if there is any--for food, until you can go no farther.

There, Carlos lives in two rooms, one of them a kitchen, with his wife, their eight kids and an abandoned child named Alexander whom they took in. Alexander is either 7 or 8--no one knows for sure.

Carlos' house is one of 11 that shares an outside spigot for water. His roof is falling in, but at least he has one. There is electricity, but no phone, a closet for a commode, two beds in the living room, a trench nearby for waste water, a television to show him that some of the world lives better and crucifixes galore on the wall to help sustain Carlos' belief that someday he will, too.

Twelve hours every night Carlos drives a cab. At the end of his tour, a man named Santos takes the cab, a Dodge Dart, and drives it for another 12. Santos lives better than Carlos does, but not much. He has six children, one of them retarded, the youngest and last of them, a girl, aged 2. Santos' wife hemorrhaged during that birth and died.

You may think that you are being introduced to two of Mexico's foremost hardship cases. Far from it. Santos and Carlos, partners in the business of driving someone else's cab, are typical. There are 16 million people in Mexico City, and 5 million of them live in some of mankind's worst slums.

Some of them have always lived there, but some are new arrivals--pushed into poverty by this country's recent economic collapse. Santos, for instance, is being forced by rising rents out of the neighborhood where he has lived for 30 years and fears that he, too, may have to live in one of the slums.

Welcome to the home office of the Ultimate Threat. If the Ultimate Threat is not the bomb that will come from the sky, but the bomb that is ticking in the Third World, then this--and not necessarily El Salvador or Nicaragua--is what we should be worrying about.

If President Reagan is right, if the threat is "foot people" and not boat people, then the so-called Lost Cities of Mexico--immense, fetid shantytowns rich in only dust and despair--will be the port of embarkation.

Here, the roads are cratered and unpaved. Here, the houses go up on someone else's land literally overnight. They are made of brick and stone and rock. They are made of corrugated steel and wood from crates and, if need be, paper. They have no water and maybe some electricity but the one thing they have--always and in great abundance--is children. The children, dirty, sometimes naked, always in rags, are everywhere.

There are kids running. There are kids skipping. Kids push wheelbarrows and carry pails of water. Kids deliver milk and they kick soccer balls. They walk in groups and alone. They ride bikes and they are carried on the hips of women. They are petted and they are hugged and they are slapped and they are hit. They either wear shoes or they don't; they either wear clothes or they don't. They will either come and share what we have, or they will not, but why they do not is beyond me.

For the moment, though, the poor have come here. They have come from other parts of Mexico, seeking jobs, and as incredible as it seems, a better way of life.

What they find is an endless commute on a broken-down bus to the city itself, air that is hardly fit to breathe, traffic that makes midtown Manhattan seem wide open, and pitiful living conditions. More than half the families live in a single room. More than half of them have more than six children.

That is more or less the life of Carlos Poo. Sitting at a table in the one room that is not a kitchen, old at 47, children climbing all over him, he tried to smile, he tried to keep awake, and I tried to imagine how someday, maybe, someone will talk about him in terms of East and West, us and them, Marxism and all the rest.

They will not understand that whatever fuels him will not be ideology, but something more basic--another room, enough food and a night's rest.