Unearthing Ancient Advanced Civilizations

by Brian Dunning
March 10, 2026

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For more than two thousand years, some people have insisted that a fictional ancient civilization called Atlantis really did exist, despite all evidence to the contrary. Others insist that even though it may not have been Atlantis, there was still some advanced race thousands of years ago, with technology we can’t imagine even today; such is the hunger of some people for alternative history. Such a civilization may have even risen millions of years ago, with all evidence of its existence having been wiped away by geological activity. It may have even been billions of years ago, wiped even cleaner by churning in Earth’s deep currents of magma. Arguments have been put forth that this cannot be disproven, such is the Earth’s ability to redact its own history. So how, then, can we establish the likelihood such an untraceable civilization might have existed?

Searching for the technosignatures of a civilization in Earth’s ancient past is not too different from searching for an alien world in space. Previously I’ve talked about what I call the “Christmas tree problem,” which illustrates yet one more barrier in the way of Earthlings making contact with some alien civilization. Imagine a Christmas tree with blinking lights scattered all throughout its branches. Each time a light is on represents the lifespan of a technological, communicative civilization. They probably don’t last long. We’ve only been communicative for about 100 years — compared to the 14 billion year age of the universe. For two civilizations to communicate, their lights have to be right next to each other, and they have to be on at exactly the same time. So even though there could be technological, communicative civilizations everywhere, the chances of any two ever being able to meet are far, far slimmer.

Now imagine one of those lights that blinks on, then turns off as its civilization dies, from any of a hundred possible causes. And then, after a bit, it turns back on a second time. That’s a new civilization having arisen — on the same planet — after the first. A hundred cataclysms might have struck that planet in the intervening spell, and its surface may have churned tectonically and been recycled multiple times. And life begins anew. It may have a completely different chemical basis than its predecessors and be entirely unlike them. But, on a multibillion-year timescale, this scenario is plausible. It may have even happened here on Earth; perhaps even several times.

This concept is what astrophysicist Adam Frank and climate scientist Gavin Schmidt named the Silurian hypothesis, in a paper published in a 2019 issue of the International Journal of Astrobiology. They borrowed the word Silurian — mostly just for fun — from Dr. Who, in which it was the name of a race of advanced reptilians who preceded human civilization. Dr. Who borrowed the name in turn from an important Paleozoic period, during which the first true terrestrial ecosystems took hold.

Though their paper was rightly presented as nothing more than a thought experiment, some in the UFO crowd picked up on it and took it as official endorsement of something they call “cryptoterrestrials.” In this conjecture, it is suggested that UFOs and aliens are not from other planets after all, but are of cryptic Earthly origins. Perhaps they live inside the Hollow Earth, perhaps they descend from Atlantis, perhaps they are an ancient Silurian civilization that has lived here for two billion years and have learned how to stay mostly hidden.

Some proponents of the cryptoterrestrial conjecture cite it as one explanation for why aliens are usually reported as being of humanoid morphology. But this doesn’t really work. We have a fairly detailed record of the development of tetrapod life some 400 million years ago, and it strains credibility to propose that this exact same cycle happened once or twice before, half a billion or so years apart, with nearly identical humanoid forms.

Let’s use the familiar analogy of depicting time on Earth like a day on a clock. Right now is midnight, and the very beginning of the Earth’s lifespan, midnight this morning, is 4.4 billion years ago, which is when all the ejecta coalesced after the collision of the two protoplanets that became the Earth and the Moon. The first upright hominids appeared at 11:58pm, only two minutes ago. The dinosaurs only existed for about an hour, from 10:39pm to 11:38pm — 22 minutes ago — when the Chicxulub asteroid wiped out almost all the Earth’s megafauna. Right around noon, for about an hour and a half, is when the Great Oxidation Event took place: microbial photosynthesis rapidly converted the carbon dioxide in the Earth’s atmosphere into oxygen. Those microbes had existed since about 1:20am, gradually becoming more complex.

At 9pm was the Cambrian explosion, a massive diversification in life that gave us arthropods and mollusks and tetrapods and all kinds of creatures; and ever since then, we have a thorough fossil record and there’s really no space for a Silurian civilization to have come and gone and escaped our notice. But we do have that big gap from about 12:30pm until 9pm. This is when there was plenty of liquid water and plenty of oxygen; 1.5 billion years when Earth was habitable, but only the most rudimentary multicellular life was around.

Could a Silurian civilization have come and gone sometime during that window? To get a better idea, let’s look at the types of evidence it would have left behind, and at the ways that evidence could have been completely wiped away.

First, here are some technosignatures that intelligent Earth civilizations (like us) leave behind for future anthropologists to see:

• Xenoliths. These are bits of stuff, like actual physical items, left over — things like plastic from synthetic polymers, concrete, and stable alloys. Physical remnants of these items would appear in the future rock strata as xenoliths.
  
  
• Synthetic isotopes. Any civilization smart enough to figure out atomic reactions will leave behind the proof that they did so, considering the example of humans. We’ll leave behind unmistakably artificial isotopes like plutonium-239 and 240 and cesium-137. However all these isotopes have half lives and will only serve as markers for a certain amount of time; some, however, will be detectable on time scales of hundreds of millions of years, like iodine-129, xenon-129, technetium-99, and a few others.
  
  
• Smog. Just as we find the atmospheric evidence of the Chicxulub asteroid in the K-Pg boundary layer from 66 million years ago, the stratum from the Anthropocene (the Age of Humans) will be full of smog. A Silurian civilization would have left a stratum similarly characterized by spheroidal carbonaceous particles from coal and oil combustion; black carbon and soot; polycyclic aromatic hydrocarbons from incomplete combustion of fossil fuels and biomass; a product of plastic dust and sediments called plastiglomerate; and — significantly — evidence of our impact on the climate, including shifts in the ratios of carbon and nitrogen isotopes.

There are lots of places where we find such records from the past besides just rock strata, such as ice cores from Antarctica and speleothems from cave formations; but of all these, the rock record is the only one with records on scales of hundreds of millions of years.

A lot of the above can actually be physically verified in person. In fact, a group of Skeptoid listeners is going to be doing just that in April 2026, on a Skeptoid Adventure to the Mediterranean that will stop on the island of Menorca. There we find strata going back some 400 million years — a nearly continuous record of the history of Earth from the Devonian period through the Pleistocene. We will see that these strata contain fossils of graptolites, trilobites, and corals — and we’ll also see that they do not contain any xenoliths. And although we’re not going to be doing a field X-ray fluorescence analysis to determine the chemical makeup of any strata, we’ll be able to see all those strata in which geochemists have yet to find any unexpected technosignatures.

And the farther back we go, of course, the less and less of the rock record survives. This is due to the forces which, as the Silurian hypothesis proposes, erase and reset Earth’s geological history. There are a lot of them, including:

• Plate tectonics, gradually dragging the Earth’s crust down into the mantle for complete geochemical recycling.
  
  
• Erosion and weathering, the action of rivers, glaciers, wind, and waves which eventually turns nearly everything into a chaos of sand.
  
  
• Metamorphism, in which so much surface geography eventually gets buried, compacted, and thermally metamorphosed into a new type of rock.
  
  
• Volcanism, burying landscapes under new rock, preserving but also obscuring it.
  
  
• Impacts like Chicxulub have local impacts on geochemistry, shocking and melting the local stratigraphy.
  
  
• Fluids and chemistry dissolve some rocks and oxidize others, transport them, and create new chemical patterns in old rocks.
  
  
• Biological activity has the potential to affect chemistry on a global scale as it did in the Great Oxidation Event, but also via underground activity like burrowing and insect colonies which dramatically alter recent layers.

Those are a lot of influences, and between them, they do a lot of damage, giving plenty of cover for a hypothetical Silurian civilization to come and go, and have its departure eventually forgotten. But would it be?

It should be noted we have fossils as old as 3.5 billion years — that’s just before 5:00am in the morning on our geological clock, when life was plentiful but still very simple and single cellular only. They linked into chains like filaments, and wove together to form bacterial mats on the floors of ancient lakes. These escaped Earth’s destructive influences and survive to be studied today.

As do countless fossils since then, more and more, and increasingly complex, with each passing millennium. All mirror the punctuated equilibrium of evolution, a ramp of function and diversity, culminating in the Cambrian explosion. The fossil record shows this ramp throughout our gap from 12:30pm until 9pm, the window available to a Silurian civilization. And yet, if they did exist, they did not interrupt the development of single celled life that is found in all available evidence - nor did they leave us any technosignatures in the strata.

On a remote shore of the Hudson Bay in northern Quebec is the Nuvvuagittuq Greenstone Belt, an endless landscape of flat, glacially-scraped rock speckled with countless lakes. Here you can pick up a stone and hold in your hand a fragment from the Earth’s earliest days when its crust had only just solidified. You can also travel west from there to the Great Slave Lake in the Northwest Territories where a prominence called the Gros Cap juts in from its northern shore. Atop the Cap you can pick up a stone, and hold a very different relict from that same opening act of the Earth, a truly primordial piece of the Earth that once was. It’s called the Acasta Gneiss. Both this rock and that of the Greenstone Belt are four billion years old, the oldest known rock on Earth. Together they illustrate that not everything gets eventually erased by geological forces; some of it survives, and some of that survives all the way.

By Brian Dunning
Follow @BrianDunning