If yesterday was a normal day, about 480 South Koreans packed their bags and moved to Seoul.

Day in and day out for the past 30 years, Koreans have been thronging to the capital in one of the great village-to-city migrations of modern history. There is no end in sight.

Somehow, Seoul has emerged as the world's fourth-largest city -- 9.5 million people, or one in every four South Koreans, packed onto 245 square miles of rocky, rolling ground.

They keep busy. One is rarely out of earshot of construction. Glass-walled office buildings, new rooms for the tiny brick houses that blanket the city's hillsides, new subway lines all are going up at breakneck speed.

Koreans do most things that way. Left in devastation of war 30 years ago, the South is today rushing through the early stages of industrialization. For the present, few people worry much over architectural beauty or air quality.

Here and there can still be found touches of the old Seoul, which became Korea's capital in 1392. There are elaborate city gates, a few traditional inns, a few contemplative ponds. But Seoul is now dominated by steel and concrete.

It has Asia's tallest building, the 63-floor Daehan Life Insurance building, and some of the most squalid shanties on the continent, too.

It has upholstered private rooms where businessmen spend small fortunes on dinners and hostesses. It has grimy video parlors where teen-agers blast away for the equivalent of 6 cents a game.

People say they move to Seoul because they have little choice. Almost everything deemed part of moving up in the world -- jobs, hot water, better schools, videocasette recorders -- is in best supply here.

"Life is easier in every way in Seoul than in the villages," said Lee Pong Ok, a rug-store employe who made the move six years ago.

Clean air ranks high in the list of things missed. Most houses are heated with charcoal. In winter, an oppressive haze over the city on many days can even be tasted.

Jobs are here because businesses face much the same choice that individuals do. Banks, suppliers, telephone lines, employes and customers are more plentiful here.

The first challenge facing the newcomer is finding a place to live. Years ago, it was possible to simply throw up a shanty dwelling and move in. Today, the land is all gone, and the government is diligent against squatters.

The old shanties slowly are being cleared away. About 10 percent of all houses here are classified as partially or totally illegal. But getting one of the apartments appearing around the city can require waiting for years and paying a price bid horrendously high by speculation.

The next battles are finding work and and a way to commute. Two subway lines are in operation, but most people use buses that creep through a bewildering maze of one-way streets, with traffic lights and construction detours.

Walking in Seoul is not for the frail: crossing its avenues, often 10 lanes wide, or, downtown, negotiating three flights of stairs every block or so to enter pedestrian underpasses are major challenges. There is no other way to cross.

Yet many people develop an enduring love for the city and embrace it as home. "If we can give them jobs," said Vice Mayor Lee Sahng Yeon, "they have a strong will to improve their lives. That's the big difference between other large cities of the world and Seoul."

Neighborhood society flourishes in thousands of small tea shops. Seoul has some of the most vibrant open markets in Asia: crowded, noisy places. In the winter, the underpasses become impromptu marketplaces, with hawkers laying out goods ranging from fresh fruit to leather baggage.

Old palaces have been turned into parks and museums. And countryside is nearby. "This is one of the few cities of the world where people can walk to mountains in five minutes," said Minister of Culture and Information Lee Jin Hie.

Seoul's size has been recognized as excessive by government leaders for years. In addition to the conventional reasons, national security figures highly in their concern.

North Korea is only 40 miles away. In 1950, its troops and tanks captured the city with most of its people before they realized what was happening. It makes little sense, goes the argument, to have so many people and factories so close to danger.

Planners at City Hall are now working on the latest city plan, this one aimed at "Year 2000."

But Seoul so far has failed to respond to plans. Its size is a byproduct of the country's push for industrialization, which continues unabated.

The city has found a strong incentive to clean up. Seoul is to host the Asian Games in 1986 and the Summer Olympics in 1988.

A multimillion-dollar spruce-up is under way.

"The Olympic Games will cost us a lot of money," said Culture Minister Lee. "But we will get a new Seoul." But not a smaller one. Seoul is expected to pass the 10 million mark sometime next year.