There have been so many that their feet have actually worn down thousands-of-years-old pyramids in Mexico.

Their sheer physical weight has triggered cave-ins on the fells of England's breath-taking Lake District.Their garbage has coated Mount Everest. Their spilled American cola has eaten into the stone floor of Notre Dame Cathedral. Their sweat and breath have covered the 2,000-year-old cave paintings of Limoges with mold and white calcite.

And yet, they remain perhaps the most sought-after consumers in all the world.

They are the international tourists, the world travelers who help make the planet just a bit smaller and their lives just a bit fuller by spending their holidays beyond their own borders and shores.

Each year now, they are counted in the tens of millions. And taken together they are the clients of what has been dubbed the largest single industry on Earth - a $1 trillion economy built solely on the pursuit of fun, fantasy and foreign adventure.

During the past two decades of well-documented explosion in the international tourism market, entire nations have planned their future economies around these international consumers. For many, it is already their single largest source of foreign exchange. And for most it has become an income so vital that their economies simply can no longer survive without it.

Tourism is so important it is often overseen by Cabinet-level ministers. Governments spend hundreds of millions of dollars each year priming the tourism pump with subsidized hotels, tax incentives and public relations blitzes. Venezuela, seeking to cash in on the South American tourism vacuum left by the drug wars and insurgencies of nearby Colombia and Peru, has launched an ambitious plan to sell off more than a dozen state-owned hotels and use the expected $1 billion in earnings to help finance construction of dozens more private-sector resorts through generous tax breaks and interest concessions. In announcing the plan, Venezuela's minister of tourism, Armando Duran, said that each new hotel room generates 1.5 jobs directly and 10 indirectly - a tempting equation for so many Third World nations mired in crushing poverty and unemployment.

Some experts predict that the international tourism explosion will continue at a growth rate nearing 10 percent per year, and analysts throughout the world are increasingly taking a more serious look at the environmental bust that the tourism boom hath wrought.

The number of potential travelers is clearly still on the increase. But many underdeveloped nations that once welcomed the international tourist as their savior are and finding that they've invited the destruction of the very beauty and culture they're trying to market.

As a result, many analysts say that the world is poised today at the cutting edge of what may well be a peaceful revolution in world holiday travel, a movement toward "gentle tourism."

Citing Spain and Greece, which have paved much of their once splendid coastlines in concrete and tar, tourism expert Graham Todd of London's Economist Magazine Intelligence Unit says, "There's an increasing realization in countries like these that they're killing the goose that lays the golden egg."

And so, the experts say, the world trend in tourism today and tomorrow necessarily will be toward the more adventurous, the more ecological and, as a result, the more expensive.

By the dozens, governments throughout the world are beginning to restrict tourist numbers. Some examples:

- Egypt, which turned Sinai beaches into urban construction zones, damaged Red Sea coral reefs through unrestricted diving and lost a chunk of the Sphinx's shoulder - possibly through tourist overuse - now restricts the number of private tombs it will open to the public each year.

- France closed its famous Lascaux caves near Limoges to all tourists after the sweat and breath of the constant stream of visitors covered ancient paintings with a veil of white calcite.

- Rwanda is levying hefty fees on every tourist visiting its gorilla country, both to finance future environmental projects and to discourage the massive influx of visitors.

- Bhutan, an isolated and reclusive Himalayan kingdom, permits only 2,000 visitors a year and insists that each spend $200 a day in an effort to avoid the cultural and physical pollution that inundated nearby Nepal, which similarly is cutting back sharply on the number of permits it issues to Mount Everest climbers.

"So, naturally, international tourism will become more expensive. It's supply and demand," according to analyst Todd. "To help save themselves from their past, many of these destinations are intentionally keeping down supply. And if demand exceeds that supply, then the prices will rise."

Many countries also are changing their overall development strategies in an effort to shift away from the "cheap tourism" that has helped foul their mountains, tarnish their treasures and poison their shores.

Brazil's Amazon Basin is perhaps the best illustration of the new, gentle wave of tourism. Last year alone, 35,000 tourists from throughout the world enriched the region with millions of dollars on "ecological tours" that include river excursions and jungle treks in which guests often sleep in hammocks or rustic floating hotels. Brazilian officials hope the Amazon adventures will eventually compensate for a 25 percent drop in tourist arrivals this year largely caused by pollution and soaring crime rates in its top tourist destination, Rio de Janeiro.