MEXICO CITY — The enormous expanse of concrete and asphalt known as Mexico City was once a lake. And each year, starting about this time, it seems hell-bent on becoming one again.
The rainy season, which begins in earnest soon, offers an annual reminder to the 20 million residents of the metropolitan area that they inhabit a big tub with no natural drain.
Flooding is common, swamping highways and sidewalks. In low-lying neighborhoods, residents are so accustomed to seeing a fetid sea of sewage rise in the streets that they have built miniature dikes in front of their homes.
"We say, 'When is it going to reach us?' " said Rafael Palomares, 72, who during heavy rains retreats with his wife to the second floor above their candy shop in the flood-ridden Iztapalapa neighborhood.
Since the days of the Aztecs, inhabitants have labored to manage the waters of the basin cradling modern-day Mexico City. Now they're trying again, with a much-touted, $1.3-billion government effort to revamp the massive but overwhelmed sewer system.
"Either we do something, or we're going to be flooded," said Ramon Aguirre, who heads the Mexico City agency that manages water and sewage.
The project involves a series of newly installed pump stations, a planned new 30-mile drainage tunnel, and repairs to parts of the 7,400-mile system of aging pipes and tunnels that carries rainwater and human waste from the city.
Officials say the fix-up -- their first good peek inside the city's main duct in 15 years -- will ease flooding by clearing blockages, patching leaks and increasing the overall capacity to drain sewage and rainwater.
Flooding might seem implausible in a landlocked city perched 7,500 feet above sea level and without major rivers. But Mexico City, ringed by mountains, lies in a basin that collects rain like a giant saucer.
Worse, the city is sinking, in some places by more than a foot a year, because it draws so much of the underlying groundwater to quench the thirst of an ever-growing population.
The Aztecs who lived here among a chain of ancient lakes chose to go with the flow, so to speak. They built a water-based society, centered on an island capital, and moved by canoe and barge along a network of canals.
The Spanish conquerors who arrived in 1519 were astounded by the vision of stone buildings rising from the shining waters. "Indeed, some of our soldiers asked whether it was not all a dream," one wrote.
The Spaniards, though, remade the landscape by filling canals to create streets and European-style plazas and later dug a huge trench to dry the valley.
Mexican governments developed the system of ducts and passages, including the century-old Grand Canal, which now can carry 2.4 million gallons a minute.
But that is significantly less than when the tunnel system was completed in 1975, even though the population has doubled. The Grand Canal, once the main sewage line, has been hobbled due to sinking. As a result, authorities have been forced to rely on a separate drainage tunnel, known as the Emisor Central, to carry the extra waste.
That tunnel, which Aguirre calls "the most important pipe in the country," has been damaged by overwork and corrosive gases that have eaten at its rounded walls, 20 feet in diameter.
In Iztapalapa, where spring heralds months of waterlogged dread, residents have heard the government's promises that drier times are coming.
Few are buying it.
Alicia Garcia, a 63-year-old retired schoolteacher, fumed as she showed a visitor how sewage, black and stinking, has invaded her family's home over the years: pouring over the window sill, surging through the grated front door, bubbling up bathroom drains.
The family has ripped up carpets, repainted walls and built a 3-foot wall where the gated entrance used to be to keep the foul waters at bay. "And the government did nothing," Garcia said.
Down the street, Yolanda Toriz recalled a flood two years ago that ruined the wood supply at the furniture shop where she works.
But Toriz, 43, chose a philosophical tack on the problem of water.
"We have to learn to survive," she said, "above it and under it."