A strange geometric fossil hides in  the sedimentary rocks of the Alps.

It's built of interconnected, hardened  tubes that weave together into honeycomb   shapes.

They spread and radiate into  larger hexagons of different sizes,   from those only a little bigger than a quarter,  to some that are over 10 centimeters across.

The pattern looks almost like the bottom  of a precisely woven basket, or a trimmed   down net - which is probably why this fossil is  named Paleodictyon, which means "ancient net."

The raised surfaces and regular, repeating  geometry are so intriguing that even   Leonardo DaVinci found them worth studying,  drawing them in one of his many notebooks.

These fossils are found exclusively in the sort of   fine-grained rocks that were once mud  on the bottom of an ancient seafloor.

But while we know where they're from,  their maker has remained… a real puzzle.

Are they burrows?

Traces of an ancient  feeding strategy?

Or maybe some sort of…   hexagon-shaped organism whose  body was preserved in mud?

That's how odd they are - we  don't even know if this is   actually a trace fossil or the organism itself.

What’s weirder is that we’ve now found  modern specimens at the bottom of our oceans,   chilling out over 3000 meters under  the sea near hydrothermal vents….

and we still don't quite know what they are.

But DNA samples and specimens pulled up  to the surface have at least given us   some new ideas about what might be behind  the modern mystery of this ancient fossil.

In 1976 scientists were exploring the  deep oceans of the mid-Atlantic ridge,   dragging a camera through the  water looking for the unexpected.

You might know these explorations for their  weird ghostly crabs and black smokers,   but that's not all that's  at the bottom of the sea.

Researchers also found thousands of  hexagon-shaped grids of regularly-spaced,   very small holes.

They looked like someone was  giving an allergy test to the bottom of the ocean.

The research team initially described them  as an "invertebrate of uncertain identity,"   and it wasn't for another few years  that the hexagon spacing of the holes   was connected to the hexagon fossil Paleodictyon.

That link was eventually confirmed with the help  of an innovative piece of scientific equipment:   a squirt gun.

Usually these deep sea vehicles have with little  water pumps meant for collecting specimens.

But some enterprising scientists  modified that pump into a tiny jet,   which the vehicle carried down to the ocean floor.

Then, they attached the jet to a big  mechanical arm and basically sprayed   the area with holes until all the loose,  soft mud flew off, revealing the hardened,   hexagonal tubes we see in fossilized Paleodictyon.

Squirt guns for science!

As a result, we now know that Paleodictyon is not  only still alive, but that it is /everywhere/.

Since the 1970s we've found modern specimens  in every ocean other than the Arctic!

In some places, they pepper the ocean floor.

And at the same time as we've been finding modern   specimens, scientists have found more and more  in the fossil record.

Turns out that Paleodictyon   has been around for over 500 million years!

Throughout that time period, they've mostly   been deep sea specialists, living quiet  lives in the dark depths of our oceans.

And the modern specimens give us a clue  as to why they prefer the deep sea.

They seem to be the best preserved and  most common where sediment builds up very   slowly - as slowly as 1 centimeter over 650 years.

It looks like Paleodictyon doesn't like  places where mud gets piled up on top of it.

Which is funny, because the way they’re preserved  throughout time is in rocks called turbidites,   also known as underwater landslides.

So we know that Paleodictyon has been   hanging around with few changes for hundreds  of millions of years.

But that brings us back   to the big question: what IS this thing?

With all these living and fossil specimens,   surely the identity of Paleodictyon would  be easy to crack… except, not so much.

Since the fossil was initially described in the   1850s there have been a few different  ideas about what it was created by.

While worms are responsible for  lots of similar fossilized tunnels,   these hexagons have such sharp angles that it  actually couldn't have been made by a burrowing   organism – cutting those precise corners at  regular intervals would be nearly impossible.

Instead, Paleodictyon appears to have  grown in place.

And there are animals   that do grow with repeating fractal patterns  in their bodies - animals like glass sponges.

These beautiful, complex sponges  build their skeletons out of, well,   glass!

Ok, silica, but it's the same idea.

Their skeletons are built with regular hexagon  shapes - and like the modern Paleodictyon,   some glass sponges live their lives  dug under the sediment.

They push up   little "spouts" above the mud surface to  help filter water through their bodies.

And those aren’t just for breathing  - sponges are filter feeders,   so they consume whatever comes along in the water.

But that's only one hypothesis.

Another option is the deep-sea   dwelling organism known as a xenophyophore.

Xenophyophores are… weird.

They're  a type of protist, meaning they’re   related to things like algae and amoebas.

But  despite being 10 centimeters or larger across,   they are actually… a single celled organism.

Admittedly, they have a lot of nuclei in  their single cell, but it's still.. odd.

They live on the ocean floor, with  some species of xenophyophores   living entirely underneath the sediment  with little spouts like Paleodictyon,   though their bodies don't form  as regular geometric shapes.

And while glass sponges are filter feeders,  xenophyophores are more complicated.

We think, anyhow.

It's kind of  hard to tell what an organism   like a xenophyophore eats - it's  not as if you can watch them chew.

But we've found evidence that some xenophyophores  absorb organic material from the water,   and others seem to eat mostly bacteria…  which some people have suggested they   might actually grow on their own poops…  Which they never poop out, by the way.

They accumulate poop in their  bodies, which feeds the bacteria,   which feeds them, which produces more  poop which… it's a living, I guess.

They also build little shells of sediment,   much like the hardened tubes of  Paleodictyon.

And they excrete   barium crystals!

There's actually a whole SciShow  episode about them, if you want to know more.

So we have two potential culprits,   both of which live strange lives at  the bottom of the sea and match the   profile.

But when scientists went ahead and  tried to get DNA evidence they found very little.

Specifically, they found NOTHING   living inside the modern Paleodictyon.

They had  apparently been abandoned – nobody was home.

And as far as lingering DNA evidence goes?

Well,   they did find some protist DNA… but it was  from other species, not xenophyophores,   and it was on the outside of the tubes, not the  inside.

But there wasn't any sponge DNA either.

They also didn't find any of the barium  crystals that xenophyophores produce.

But their efforts weren't entirely a  bust.

See, these modern Paleodictyon   have something that fossil ones rarely  have preserved: those little spouts,   which formed the holes that the scientists  spotted on the surface in the first place.

And on closer examination, researchers have  found that the pipes aren't of even heights.

The ones in the middle are  slightly taller than those   on the sides - which means that when  a current passes over the hexagons,   water gets forced down into the  pipes more than it otherwise would.

This would help refresh the water  inside Paleodictyon's tubes,   and it would force nutrients and any too-slow  swimmers down into the fossil.

Which would make   the hexagon net a literal net – good  at trapping animals for consumption.

Now, to some scientists,   that makes it seem like Paleodictyon should  be a sponge – that's how they eat things!

But other researchers have pointed out that the  repeating fractal pattern of Paleodictyon is   actually typical of organisms  that have a different lifestyle… Specifically, of organisms that need a perfectly  regular pattern to get exactly the right amount   of water and oxygen flowing in a system –  as if, perhaps, they were doing some sort of   hydroponics thing to farm their poop bacteria,  the way we think some xenophyophores might do.

And sponges usually leave their  skeletons behind – they don't make   hard sediment tubes the way xenophyophores do.

Ultimately, both explanations  seem plausible, but unproven.

And until we find a real living organism in one of  these things, the debate will continue to rage on.

Paleodictyon is a really strange case.

It's a bizarre fossil to start off with,  but the fact that it remains unidentified   really speaks to how much science is  still out there, waiting to be discovered.

And it's definitely not the only mystery  lurking at the bottom of the ocean.

As one scientist pointed out, the  only reason we found Paleodictyon at   all was because it’s a sequence of geometrically  spaced repeating holes.

That kind of stands out.

Many other similarly strange trace  fossils might have modern versions,   but we don't find them because they’re made by  organisms much more secretive and less showy.

If Paleodictyon only had a single spout, we  might never have known it was there at all.

Which is one of the amazing things about science.

Who knows what the next dive will bring us?

Not Megalodon, that's for  sure.

But, maybe someday,