The United Nations has designated roughly 1,000 culturally important World  Heritage sites around the world.

But at many of them, something  has quietly been going wrong. Paintings are flaking away, wood is rotting, even stone is slowly dissolving into powder.

If you’re wondering what’s so  powerful that it could take down some of our oldest and most protected sites… well, this destruction isn’t the  product of war or natural disasters or other huge events. It’s the work of microbes. Here’s how some mini microbes  are ruining our ruins. [♪ INTRO] First up, we’re going to the Maijishan Grottoes: a series of roughly 200 caves carved into the side of a sheer sandstone mountain in the  Gansu province of northwest China.

You have to be feeling brave to cross  the soaring walkways that lead inside. But once you’ve entered a grotto, you can find sculptures and wall  paintings from as far back as the 300s. But despite standing through 1700  years, the site is in trouble.

In 2018, experts reported that microbial  plaques had started growing on the art! As far as we know, it’s not like  microbes hate art or something. They do this to make a home for  themselves or to get nutrients.

In fact, while a painting might seem  like a weird place for life to grow, if you’re a microbe, paintings  can be full of tasty bits. There’s the paint itself, which  might contain things like plant oils, egg yolk, or animal glue. But there may also be layers of organic materials like straw or other plant fibers beneath as well, which the ancient painters would have slathered on the wall to give themselves  a smooth surface to work on.

That is a balanced meal for a microbe! As for what kind of microbes, there’s  a wide variety of bacteria and algae. Both of those organisms can do art-damaging things like poop out acids or form gooey  biofilms that can trap water.

You can also get molds and other  fungi growing on those ruins, which spread their root-like hyphae  underneath the surface, looking for nutrients. All these little fellas can physically  separate the layers of a painting, causing bits to flake or fall off. They can also cause changes in color, either by degrading existing pigments or  by adding their own colors into the mix.

Green algae can turn things, well, green. Certain bacteria can do green as  well, plus yellow, brown, or black. One kind of bacteria can even  create a lovely salmon pink.

But in the case of Maijishan, there were mainly white and black  splotches appearing on the walls. When scientists analyzed the splotches, they found that a common decomposing fungus was the dominant microbe in the black biofilms. Meanwhile, Cladosporium, a  super common indoor mold, and an unclassified species of fungus  were dominant in white biofilms.

As for what caused the outbreak,  they don’t know for sure, but it probably had to do with cave critters. The decomposing fungus is associated  with arthropods, like bugs. And fungi can also grow on  animal dung or after flooding, so animals getting into these caves, to shelter from heavy rains,  might be contributing.

In fact, previous work on the splotches  also pointed fingers at excess humidity as a possible cause and suggested  it could be from more human visitors or even shifts in the weather  thanks to climate change. We know this is possible. The Lascaux caves in southwestern  France contain wall paintings from as far back as 22,000 years ago.

But after their re-discovery in the 1940’s, lots of people began visiting,  and the carbon dioxide, heat, and water vapor from their  breath caused microbes to grow. The authorities eventually closed the caves  to the public and for forty-ish years, tried using biocides, including  antibiotics, to get rid of those microbes. And it worked on some of the microbes.

But that opened up space  for a new fungus to move in, which left a whole new set of white stains behind! Similar things happened when  people tried to disinfect the frescoes of St. Paul in Ephesus.

So biocides don’t always work as well as we hope. Another option is manual removal,  like using soft brushes, a vacuum, and a little bit of elbow grease. …Which  they tried in the Maijshan Grottoes, along with biocide treatments. Those measures seem to have worked well, even though this is probably  going to be an ongoing project.

But paintings aren’t the only type of  heritage under pressure from microbes. We’ve also had some problems with wood. But before we go there, it’s time for a quick ad.

Thanks for watching this SciShow video! And thanks for supporting SciShow by commenting, sharing, and joining us on Patreon. There’s a lot of online ways to  keep fact-checked information, like what you find in these videos, around.

And now there’s a way to do that in person too! Complexly, the 501c3 non-profit  that makes SciShow, PBS Eons, Crash Course, Bizarre Beasts, and more,  is holding our very first benefit! It’s called Something Worth Knowing and  it’s live at the Wilshire Ebell theatre in Los Angeles California on September 16th, 2026.

This first-ever Complexly in-person  event is a fundraiser to support our mission of inspiring curiosity and  lowering barriers to knowledge building… because curiosity builds understanding,  and understanding builds a better world. You can get your tickets at  somethingworthknowing.org. See you there!

In the desert of New Mexico,  you can find Chaco Canyon. It’s another UNESCO World  Heritage site, occupied by the Ancestral Puebloan people for about 300  years, starting around the mid-800s. While they were there, they  built impressive great houses and other infrastructure out of stone and wood.

And so far, it’s stood the test of time. But not entirely unscathed. For example, in the mid-oughts, experts  examined some of the wooden beams that made up the buildings and found  a significant amount of damage.

Some was from weathering and  other physical processes. But some was from microbes. Now wood, especially wood in  harsh, dry environments like the New Mexico desert, can be pretty  resistant to microbes like bacteria.

Wood is tough and durable because plant  cells are surrounded by cell walls and woody plants in particular  pack them especially full of tough, hard-to-break-down molecules  like cellulose and lignin. But wood is vulnerable to  fungi, because they have ways to get into and even get  food from these compounds. One of the main trouble makers is a group  of fungi that includes brown-rot fungi.

These little guys specialize in  decaying the cellulose in plant cells, leaving the colorful lignin behind. This turns the wood soft, but leaves  the wood’s brown color behind. And that’s where they got their name.

White-rot fungi, meanwhile, turn wood white since they decay everything, including lignin. Some even specialize in lignin. Another major kind of fungal  wood decay is soft rot.

This looks really similar to brown-rot. In fact, it’s so similar that you  generally need to look at samples under a microscope to tell  soft-rot and brown-rot apart, even though the organisms behind  the rot are pretty different. And experts have found some brown-rot and  a lot of soft-rot setting in at Chaco.

Some parts of the wood were so degraded they could be crushed into a fine  powder with even slight pressure. Part of the problem may have  come from fissures in the wood, which can catch moisture and nutrient-rich  particles from the environment. The perfect conditions for microbes to move in.

In this case, instead of  biocides, we’ve been reburying parts of the site since the 1980s. The idea is that digging  them up likely exposed them to the moisture the fungi needed to thrive. So reburying them should hopefully remove  that moisture and stop further growth.

As an added layer of protection,  conservationists also installed things like moisture monitors  alongside the buried material, as well as waterproof fabric  sheets or drains and pipes that can funnel surface water away from  stuff buried further down. Reburial is pretty low-tech, practical,  and, compared to trying to replace the wooden beams or other materials again  and again, a good deal money-wise. We like to imagine technology  solving all our problems, but sometimes the best option for archaeologists is to leave things alone and  preserve them for the future.

We can get the technology to fix it later. Although eventually everything  breaks down, whether it’s paint, or wood, or even stone itself,  as our last location shows. In 2018, scientists took samples  from a bunch of sites in Egypt, from tombs and obelisks to mosques  and even the pyramids at Giza.

These sites are ancient, and  have withstood the test of time. But over the years, people have  noted damage at some of them, including black stains and  pitting in the rocks themselves. Scientists found two species of a common mold along with Cladosporium, one of  the fungi found at Maijishan.

And we’re not just seeing it in Egypt. Another desert site, Megiddo,  was surveyed in 2023. This world heritage site was swabbed from the visitor entrance to its  major archaeological sites.

Experts didn’t report damage, but  did detect acid-producing microbes. As for what a microbe would even want  with stone, just like wood or paintings, it can give them a home and even food. They might even damage the stone  itself by doing things like dissolving minerals from the rocks,  which unfortunately for UNESCO, can be especially damaging  in rocks like limestone.

Plus they can cause damage through things like those watery biofilms and root-like hyphae. And in some cases, they can even poop out salts, which can be destructive,  both chemically and physically if it starts to build up in  cracks and force them apart. Case and point, the Egyptian  group applied Cladosporium fungi to some fresh stone and found  it discolored and weakened.

It had even dissolved some of  the stone after just two months. To treat those issues, they tried  a bunch of different biocides on their stone blocks and found some that worked, including synthetic topical antiseptics,  as well as natural options like clove oil. But we don’t want to go indiscriminately  killing all life at these UNESCO sites.

Because in some cases, microbes  might actually protect them. At Angkor Wat in Cambodia, for  example, scientists found that lichens might help protect the stone  buildings there from water damage. They were effectively shielding  the site from the weather and may also have been pushing out other microbes  that could have damaged the site.

Now, lichens actually a  mix of microscopic bacteria and algae living inside a kind of fungal web. And some of them are microscopic. So they might toe the line of microorganism, but for the purposes of this  video we’re counting them.

But if you’re not Team Microbe  when it comes to lichens, there are still other protective  guys under the microscope. Some scientists are even experimenting  with the idea of purposefully applying certain harmless  bacteria or amoebas to different archaeological sites as a preventative  or even restorative treatment. In plaques and other microbial communities, including ones from places like murals or ruins, there are protective microbes that may crowd out, chemically suppress, or directly  prey upon the ones that cause damage.

There are even microbe-targeting  viruses we might be able to use. One group experimented with using  sulphate-eating bacteria to remove unsightly black, sulphate-rich  gypsum deposits from the base of a famous marble sculpture,  Michelangelo’s Rondanini Pietà. The bacteria were mixed into a gel that  could be slathered onto the sculpture.

A day later, scientists came back, washed it off, and found that the deposits were gone! So along with all of the publications coming  out that highlight microbes’ destructive ways, people are experimenting with plenty  of them that could help in preservation. We often talk about these sites as  if they’re kind of frozen in time.

But, in fact, they’re  constantly changing ecosystems. Microbial research helps  us understand those changes so we can preserve the sites  for generations to come. Sometimes if you want a better view of the future, you need to get out a microscope. [♪ OUTRO]