You can't possibly grasp the numbers: 3,729,42 maps, 44,385 atlases, 750,000 fire insurance plans covering 12,000 American Communities'

You begin to get a sense of it when you stare -- gawk is the word -- down the underground corridor that runs the whole length of the new Madison Building, 640 feet (two football fields laid end to end), its perspective lines virtually swallowed up in the hazy distance, and all the way down on both sides you see these long rows of metal map cabinets, four feet high and four feet wide, 6,000 of them. Full of maps.

Some of them are muddy-colored, cluttered tools for technicians. Others are gorgeous: conceived with clarity, executed with precision, created with an artist's love.

"We have probably the most comprehensive and certainly the largest collection in the world," says John A. Wolters, chief of the Library of Congress' Geography and Map Division.

He riffles through a pile of stuff that "came in the last week or two." The pile is stacked a foot high on a large table. A 1791 street map of Prague (in Czech) is on top. A road map of France. A Harrisburg, Pa., street grid. One of those National Geographic fold-out maps. Tourist maps covered with cute drawings. A huge chart of all the pools and dry holes in Oklahoma. The city of Seoul. Nautical charts.

"We're doubling our collection every 23 years at the present rate. We acquire 150,000 items a year and select half of them. And every year our summer interns comb out 60,000 or so maps for their universities."

If maps proliferate like mayflies, they also blow away with the first breeze. Last year maybe 200 million road maps were published in this country; 20,000 made it through the whole year; within five years fewer than 1,000 will survive. Which makes all the more remarkable the collection's oldest map, the eastern Mediterranean somewhere around 1320-40.

Like all early nautical charts, it is concerned mainly with point-to-point guidance of ships. The only cities shown are the ports, and these appear only as names written perpendicular to the shoreline. You make out the shape of the land from those names, more or less incidentally.

There are other priceless maps, of course: maps showing California as an island, a misinterpretation that arose years after its discovery and persisted for generations; Champlain's own handmade map of his 1627 exploration of the New England coast, literally drawn as he sailed it; a globe showing the earth-centered Ptolemaic universe, made in 1543, the same year Copernicus shattered that universe; a 15th-century chart of weights and measures used by Mediterranean commercial cities; Chinese scrolls of a rather vague ancient world; maps on fans and globes and powder horns. The variety, the scope of this collection are literally beyond description.

"Map design is a real art," says Wolters, "not just in terms of those lovely old parchments with their drawings of the ships and costumes and measuring instruments of the day, but now, in the emerging discipline of cartography."

Since World War II, he notes, there have been two revolutions: computer techniques and remote sensing by satellite, which permits, for instance, a mapmaker to depict healthy farmlands and depleted acreage on a worldwide scale.

Since moving into town last year to its efficient new quarters, the map division has been getting even more visitors. Most of them are less interested in how the world looks than how it used to look; they are genealogists.

And have they ever found a Happy Hunting Ground. Here are the first atlases ever done of nearly all the states (the first was South Carolina, 1825). Here are cabinets and cabinets, volumes and volumes of fire insurance maps, beginning around 1850 and updated every decade. Your hometown was Clinton, N.Y.? There it is, in 1874, with each building and property outlined, the building material and method of heating specified. There's your farmhouse, and the corner store. There's the old mill on Franklin Street (one circular saw, one planer, 24 boilers, iron roof truss), and up on College Hill each structure of Hamilton College is drawn to scale, right down to the firewall in the chemistry lab. On the next page you see how it looked in 1884.

There are maps like these for 12,000 cities and towns.

Including Tombstone, Ariz., where you can find, down behind the saloons and "boarding houses," a small open yard marked: O.K. Corral.