Tourists throw their coins in the Trevi Fountain in Rome. Photo / AP
Leonardo da Vinci's Last Supper, the Trevi Fountain in Rome, Stonehenge, the Hermitage Museum of art treasures in St Petersburg are all buckling under the weight of a global tourism boom.
And in response to the onslaught, the countries that own these wonders are removing them from view, or at least erecting barriers or restricting access to protect them.
Under the stampeding hordes of curious travellers, not to mention drunk tourists, these irreplaceable antiquities have been chipped, cracked or threatened by mere breathing.
In the Italian city of Milan, for example, Leonardo da Vinci's's great masterpiece The Last Supper is still open to small private tour groups to view with the naked eye.
But not for long. The faded 1498 fresco on one wall of the Dominican monastery refectory by the Santa Maria delle Grazie church is about to be put behind a wall of plexiglas.
Kept in a climate-controlled environment, the refectory can be visited by a maximum of 25 people every 15 minutes whose very breath raises the interior to room temperature.
But news.com.au has learned that due to fears that this priceless treasure is deteriorating, a transparent wall will be erected to seal it completely from human contact.
Putting it behind plexiglas will dilute the experience, but Milan is not alone.
Just this week, Rome's mayor Virginia Raggi has signed an order to protect of the city's most precious monuments including the famous Trevi Fountain.
It is now forbidden to sit, climb, eat and drink, let alone frolic or swim in the Trevi, though you can still follow tradition and throw in a coin — but nothing else, or risk fines of up to 240 euros ($355).
Bernini's fabulous marble fountain has featured in many films, including Audrey Hepburn's Roman Holiday, and some tourists have been moved to frolic in it like Anita Ekberg did in La Dolce Vita.
Raggi signed the order over more than 40 places of historic or artistic worth which have been vandalised or soiled over the years.
In 2015, Dutch soccer fans damaged the 17th-century Barcaccia fountain at the foot of the Spanish Steps and littered its waters with broken beer bottles. This spring, a man swam naked in the Trevi Fountain.
"Everyone must respect Rome's beauty," Raggi said.
The prehistoric monument Stonehenge, which lies on the Salisbury Plains in England's Wiltshire has been on non contact visits for years, apart from special access visits sold months in advance.
But while it was viewable from behind a rope fence, the new visitor centre allows more visitors but puts them further away from the stones.
PACKED IN LIKE SARDINES
The Hermitage Museum in St Petersberg holds one of the greatest collections of art and historical pieces in the world.
But, much like the Sistine Chapel in Rome's Vatican, it has become so packed with visitors that it is hard to enjoy, let alone spend properly seeing individual items.
Especially large tour groups, such as passengers on the enormous cruise ships which dock at Russia's old capital on the Baltic Sea, have forced the Hermitage to limit access to only a portion of the museum.
And at Brú na Bóinne, the prehistoric passage tombs in Ireland that are older than both the Egyptian pyramids and Stonehenge, access may soon be a thing of the past.
OLDER THAN THE PYRAMIDS
Built in 3200BC in the Boyne River Valley north of Dublin, three large burial mounds called Newgrange, Knoweth and Dowth were made a UNESCO World Heritage site in 1993.
Only the interior at Newgrange is still open to the public, and that is under threat.
Believed to have had religious and astronomical significance, Newgrange's entrance is aligned with the rising sun on the winter solstice.
Its stones feature mysterious carvings of spirals, circles, zigzags and triangles.
At 8.40am of December 21 after the Northern Hemisphere's longest night, sunlight shines through a "roof box" and floods the inner chamber, signifying midwinter and rebirth.
But news.com.au has learned that after this Northern summer, Ireland's Office of Public Works (OPW) is seeking to restrict numbers of tour groups permitted inside the ancient mound.
Tour company operators who enjoy hard-won access to the site have been restricted from accompanying their groups.
The OPW has now announced it will allow fewer and smaller groups and it is unclear if access will eventually be blocked.
Bru na Boinne Visitor Centre presents a display of the Boyne Valley archaeological sites.
But will replicas be made for tourists like at France's Lascaux Caves?
FAKE BUT STILL FABULOUS
Located in the Dordogne region of southwest France, the Lascaux Caves with their 18,000 year old prehistoric paintings caused a sensation when boys happened upon them in 1940.
Tourists were banned from the original site in 1963 because the carbon dioxide they exhaled damaged the paintings and wreaked havoc with the cave's fragile ecosystem.
A copy of the site Lascaux 2 opened in 1983, but reproduced only a percentage of the cave's wall art.
Last December, the French Government unveiled a new replica of the cave paintings to recreate the experience of the four teenagers who found them after exploring a hole in the ground in the hills above their village.
Tourists are undaunted by the fact they are seeing a fake, and as France's then President Francois Hollande remarked as he entered the display, "this is more than a copy, it's a work of art".
IT'S NOT THE SAME
But for some, touring a replica or being put at arm's length from the real article diminishes the return of visiting one of the world's truly great wonders.
Many visitors to The Last Supper report the "emotional" experience of being in the same room as the Renaissance genius da Vinci's iconic work.
While this move will not deter the globetrotting multitudes beating a path to its door, it may make global sightseeing less meaningful.
WORLD'S MOST FAMOUS PAINTING
Da Vinci's even better known work, the Mona Lisa is the world's most famous painting.
It has been behind glass in the world's greatest art museum, the Louvre, for decades.
Leonardo da Vinci painted it between 1502 and 1506, and it hung in royal palaces until it was placed in the Louvre in 1798.
It has been stolen several times and is fragile despite delicate preservation.
Daily traffic jams of tourists and 6.6m annual Louvre visitors a year visit mostly to see the Mona Lisa, or be photographed in front of it.
Small, at just under 77cm by 55cm, "La Gioconda" with the legendary mysterious smile hangs behind 3.86cm glass in a permanent temperature of 6 degrees Celsius and 50 per cent humidity.
It will always be great for a selfie, but many are disappointed by the experience.
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