VENICE, Italy — The warning siren howls in the dark.
As always, the flood arrives on the tide.
Water slops over the rim of the city's canals, bubbles up through drains and, as the morning hours progress, surges into the ground-floor vestibules of palaces along the Grand Canal. It leaves stripes on sills, green and brown with a sticky scum of sewage.
The wind-driven high water -- acqua alta -- is a staple of Venetian life, like an espresso and a croissant con marmellata at a stand-up coffee bar.
Porters in knee-high waders set out elevated walkways. Shopkeepers slide cofferdams across doorways. Gondoliers duck their heads as they pass under bridges arched over swollen canals.
Brackish high tides swamp the city about 100 days a year -- twice as often as 50 years ago. After a thousand years of adaptation to the water, Venice is foundering.
"There are no longer any homes at ground floor," says Jane da Mosta, who lives with her family near the Grand Canal. "Nobody risks being drowned in their sleep."
Ge Beaufort, 60, a senior hydraulics engineer at Holland's Rijkswaterstaat ministry, shades his watery blue eyes to admire the view.
Along the broad estuary of the eastern Schelde, southwest of Rotterdam, Dutch engineers made their stand against the North Sea.
It took $3.2 billion and 19 years of construction to impose their schematic on the greenish-gray currents of the estuary.
Braced shoulder to shoulder, 65 concrete piers of the Oosterschelde storm-surge barrier tower above the churning tide in a titanic picket fence that disappears over the curve of the horizon. Each pier measures up to 100 feet high and weighs 8,000 tons.
The 5 1/2 -mile storm surge barrier of the Oosterschelde is the backbone of national coastal defenses called the Delta Works, so vast they can be seen in their entirety only from space, so crucial to the survival of the Netherlands that the government spends nearly $2 billion a year to maintain them.
Beaufort stands on the beach of a 2 1/2 -mile-long island dredged from the sea simply to hold the project's construction machinery.
"It is an engineering monument to the most the human imagination can do to control nature," he says. "On a higher level, it all is only a small toy."
These are the water worlds.
For a thousand years, the people of Venice have let the tide wash through their city on the Adriatic Sea. For just as many centuries, the Dutch have pushed back the sea.
One lowland built an empire on reconciliation; the other on resistance.
They have warded off the water as no place else in the world. To survive, these two coastal centers seek stability along a tide line where geology, geography and the powerful whims of water offer none.
In their long struggle, all they have gained is time. Now, time may be running out.
Alarmed by the growing intensity of storms and rising seas, each of these urban lowlands is at a turning point in its struggle for the control of nature.
As planners and engineers calculate the cost of resurrecting New Orleans from the damage of Hurricane Katrina, the sobering lessons of Venice and the Netherlands figure prominently.
The Louisiana Recovery Authority estimates that it could cost $20 billion over the next decade to rebuild and strengthen the 350 miles of levees encircling the city.
It is only a fraction of what Venice and the Netherlands have spent in centuries of struggle with the sea -- and must continue to spend for as long as their ingenuity can keep nature caged.
In the century to come, global warming is expected to raise sea level between 4 inches and 3 feet. Already, the destructive power of North Atlantic storms has doubled since the 1970s. Unchecked development in the coastal lowlands compounds the growing natural hazard.
All told, more than 40 countries depend on dikes to protect coastal settlements. Nearly 50 million people live within reach of a potential storm surge.
In London, a $1-billion storm surge barrier on the River Thames stands between high water and the 60 square miles of the central city below the tide line. Since it became operational in 1982, it has been closed 83 times to prevent flooding.
This year, engineers from the Stony Brook Storm Surge Research Group urged construction of three mobile storm surge barriers to protect New York City, where a 1992 storm surge shorted out the subway system, shut down LaGuardia Airport and left parts of Manhattan under 4 feet of water.
"At the end of the game, nature does what nature wants to do," says MIT civil engineer Rafael Bras, who heads a panel of advisors to the Italian government on water management in Venice. "Disaster strikes at its own pace."
Set at sea level in a shallow lagoon, Venice was designed by its founders to harmonize with the water, from the millions of alder pilings that anchor its grand palazzi in the muck to the maze of 177 canals that serve as streets and sewers.