People have such a low opinion of the feats of engineering that our ancestors were capable of that they invent all sorts of magic explanations.

Giants, cyclops, menehune, elves, and such have been invoked frequently to "explain" the achievements of past humans. Now aliens are popular because apparently past humans didn't have brains, manpower, and time.

I think it was David Brin who wrote a short story and a foreward explaining that aliens have taken the place of faeries in our culture.

People have made stone spheres with primitive means already. See: https://en.wikipedia.org/wiki/Stone_spheres_of_Costa_Rica

I thought the Menehune were just considered to be other people who got there first...

Indeed, it should be noted that our technological civilization has been developped very quickly, in basically two or three hundred years (and we're on the verge of self destruct thru nuclear weapons, of which there are signs or legends of prehistoric use all over the planet).

It is certain that our ancestors have had more than enough time to develop powerful technolonologies several times over, and we have remnants of their technology all over the planet too (the pyramids and other megalithic constructions being only the most visible, since the most sustainable).

The use, and more importantly the development, of nuclear weapons leaves distinctive isotopic signatures that last for literally billions of years. In fact, an ancient naturally occurring "fission reactor" that is 2 billion years old has been discovered in this way. There is no other evidence of such activity in that time frame, a time frame that includes the history of all multicellular life on the planet. We can, with fair certainty, rule out the existence of any previous nuclear capable civilizations on Earth.

playing the advocate of the devil here, maybe the civilizations cleaned it all up and went solarpunk just before they transcended and vanished...

Basically an impossible task, and why would they?

Imagine how hard it would be to completely cover up the evidence of human existence on Earth, for example. You'd have to find and excavate every single arrowhead out of the Earth, every single bit of plastic from the ocean. You'd have to figure out something to do about all the mines. You couldn't just fill them up, that would leave evidence. And even that would just be scratching the surface.

Creatures complex enough and with big enough bodies and brains to potentially evolve into something that might build a technological civilization have only been around on Earth for a couple tens of millions of years, which isn't very long in geological terms. We don't see any evidence in the fossil record of ancient big brained creatures developing, and that's pretty much a pre-requisite for a technological species. There aren't any candidate creatures in the fossil record that could have conceivably evolved into a technological species in only a few million years, which means it would take a very big gap in the fossil record to have zero clues about such a thing, if it did happen. Also remember that fossil fuels are part of the fossil record, so we can be pretty confident that they didn't use fossil fuels.

And, as mentioned, if they'd ever developed nuclear technology we would know about it, because the isotopic signatures from either their mining activities, enrichment activities, the operation of their reactors, or the above ground testing or use of nuclear weapons would be like a beacon.

So, absent Slartibartfast levels of world-making or re-making, it's not a plausable theory, even as a stretch.

Overall it would only make sense if they never used fossil fuels, never mined, never developed nuclear technology, never cut down forests, never had farms, never left behind artifacts made out of bone, metal, stone, ceramic, glass, plastic, etc, never modified the landscape in any way. And never had any evolutionary predecessors that left a fossil record. If you can figure out how to squeeze a plausible technological civilization into those constraints, good luck.

Second time I get to mention David Brin in this submission!

You might enjoy his Uplift series; it deals with scenarios not unlike what you mention.

> nuclear weapons, of which there are signs or legends of prehistoric use all over the planet

Er, are there?

Well no, but I remember a long time ago reading some theory about "red indians" having atomic weapons and it was somehow linked to ORME (http://rationalwiki.org/wiki/ORMUS) - absolutely crazy, yet entertaining.

It seems reasonable to assume that any earlier iterations of a global civilization would also have relied heavily on the burning of coal and oil for energy, at least for a while. And yet there has been no spike in atmospheric CO2 concentration in the last 400,000 years like the one we have now:

http://www.geocraft.com/WVFossils/last_400k_yrs.html

What "powerful technology" are the pyramids evidence of? Slave labor? Ropes and logs?

The pyramids were probably massive public works projects, with off-season farmers making up the workforce, paid in beer. Not slave labor.

Actually, a metal machinist from the Midwest named Christopher Dunn has a fascinating theory that the great pyramids may have actually been 'power plants' of a sort.

Giant harmonic resonators that siphon energy off of the earths expansion and contraction caused by orbit of the moon.

http://www.amazon.com/Giza-Power-Plant-Technologies-Ancient/...

His writing is a bit dense, and maybe not as smooth as it could be...but his exploration of the possibility is quite impressive.

As you're by all appearances unaware of the concept, a bit of light reading...

https://en.wikipedia.org/wiki/Pseudoscience

and https://en.wikipedia.org/wiki/Scientific_theory (with an emphasis on the "well substantiated" bit)