If we could temporarily nullify/reverse gravity would we all be weightless? Is that how the ancients moved incredibly heavy objects?

Actually a lot of what they did achieve has been duplicated by modern archaeologists experimenting with the resources likely to have been available.

The ancients made considerable use of the inclined-plane (ramps), levers (probably lengths of tree-trunk), rollers (ditto) and ropes (hide or vegetable-fibre). And large gangs of labourers! Yes, it was slow and sheer hard work, but it was the only feasible way.

There are still some puzzles, such as how ancient South American civilisations managed without the wheel, and my guess is that they used roller, but had not made that logical jump from roller to wheel.

Another is how they succeeded in moving very heavy blocks of stone across terrain where rollers would sink into the ground. They appear not to have made hard roads for this. So again thinking laterally, they could very easily have laid lengthways timbers to carry the load-bearing rollers. If so, a curious precursor more to the railway than the road.  

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In fact such simple tools plus physical man-power persisted in some applications until well into the late 19C, before suitable high-lift cranes became developed. Erecting the huge vertical steam-engines first used in the Cornish tin-mines, and similar but more refined ones later built for water pumping-stations in places like London, entailed raising very heavy components to quite considerable heights. All they had were suitable beams in the engine-house roof for suspending lifting-tackle. The work was by team effort: the engine manufacturer's fitters bolted the parts together, but much of the lifting was by former seamen very skilled at using no more than ropes, blocks and tackle, knots and lashings, and simple windlasses, to lift and move very unwieldy items at height.

The famous Iron Bridge at the English Midland village of Ironbridge, is the world's first iron bridge (18C, and before steel was developed). How its prefabricated parts were assembled, across a small gorge carrying the fast-flowing River Severn, puzzled modern engineers. Then someone chanced to spot a contemporary painting by a Swedish artist, showing just enough of the initial phase of its assembly to make sense of scanty references in surviving financial accounts for the building, hence an experiment to prove the technique.

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A final example, but a small-scale, personal so modern one. Helping to load a heavy machine-tool onto a trailer, the improvised ramp collapsed, toppling the machine into an adjacent, very dense blackberry-bush. Luckily no-one was injured. Now what? First thing, move the trailer out of the way and turn the machine back upright.  Outdoors so no overhead lifting-point. No crane available. No direct access to the other side of the machine (If the blackberry is not native to your part of the world, think barbed-wire entanglements and then some.) Simple: ropes, a long strong pole and several people. Lashed the pole vertically to the machine's base, with a rope tied to the top, so giving us the leverage to pull the machine to its point of balance. Extra ropes in the other direction, then controlled its swing to proper upright position.  

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It is very easy for us seeing modern machinery in action to ask the wrong question: "How could they have done that without...?". We have instead to think, "What did they have instead?", and try to see the problem as they did.

They had very simple lifting-and-moving tools still often useful today, lots of man-power, sheer persistence, rather more time that we allow ourselves... and the same level of intelligence, lateral thinking and creativity as the modern Engineer. 

Well, no-one has yet managed to deduce what gravity really is, and there is no evidence that past civilisations were so far advanced beyond ours that they could have cracked that problem and designed some form of levitation! If they had the remains of their equipment would still be around.

Of course there may have more trees within reach at the time, but the residents deforested the area beyond recovery. Or simply brought the materials in from elsewhere.

Nor have any man-made individual objects, such as building-stone blocks, weighing millions of tons ever been found, that were moved to their locations. Several tens of tons, yes; occasionally several hundred tons. I think the contributor confused total building mass, with individual block mass. Or carried out no proper research.  

To estimate what a million-ton block of rock looks like, I carried out a simple calculation, but I chose to use basalt. It is about the most dense of the common rocks, but its outcrops are restricted to certain volcanic regions and it also very hard so I don't think was ever used in large-scale buildings of any sort; and certainly not in very large pieces. I used it to give a rough minimum size of a single rock cube weighing 1 000 000 tons.

The figures I found are of course metric but I'll give approximate equivalents.

If a cube, one-megatonne (not significantly different for our purposes from a million English or US tons) of basalt is roughly 70 metres (230 feet) along each side. So hardly compact. That's basalt, averaging 3g/cm^3. The rocks far more commonly used since ancient times for masonry are limestone and sandstone, much softer, and rather less dense than basalt so the 1 Mt cube of them would be even bigger.

Ancient peoples have carved homes or other "buildings" out of single masses or cliff-faces of limestone and sandstone; and these might occupy similar volumes - but no-one has moved a single lump weighing a million tons and over 200 feet across each way! 

The arch-stones of Stonehenge's trilithons average about 800 tonnes. A very impressive load to move and raise, indeed; and it would tax modern engineering methods to copy; but it is still far, far lighter than 1 megaton! This and its companion monuments stand on Chalk down-land devoid of trees for a long way around, except in the valleys; and it's feasible their construction and other human activities since, deforested large areas to the point soil-erosion and agriculture ensured the trees would not come back. 

If anyone in the past had defeated the Laws of Physics so comprehensibly they had left structures whose individual components are of Mt mass, not only would the remains be as famous as the Pyramids, Macchu Picchu or Stonehenge (all using far smaller parts) but we would also expect to find significant traces of the construction methods and equipment.

You say you forget the site or location mentioned. Did they actually mention it at all?

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This rather reminds me of Eric von Daniken. Remember him? He was a Swedish hotel chef who wrote books fantasising about his scanty or non-existent visits to "study" ancient artefacts or even entirely natural features, and "explain" them. Others looked carefully at the sites concerned and very rapidly dismissed his notions as fantasy and laziness, if not sometimes, outright lying. 
X This post was edited by Durdle at August 5, 2019 3:52 AM MDT

Oh come now Rosie! I'm sure none of it was over your head, but to summarise, "Megaton" means "One Million Tons" and I calculated the minimum size of a block of rock that heavy, to see just how big it would be.

The answer? Far, far too big and heavy for anyone to have moved in one piece.

Just occurred to me, but not knowing the location I can't assess this: If it was on a steep hill-side, I wonder if the mass of rock talked of had actually been moved by a major land-slip or other geological process, not recognised by the people who saw it.

By the way when I said the archaeologists had duplicated ancient buildings, I don't of course mean entirely. They duplicated the most feasible way the builders could have moved the building's parts, by experimenting with identical copies of those.

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I think the greatest of ancient building achievements are not those even of the Egyptian Pyramids and the like. Most very old buildings except those of the Romans were quite crude, put together by some skill, a lot of common-sense and a great deal of sheer hard work.  The Pyramids were built with great care and skill, but are still basically small rooms and corridors made by leaving gaps inside self-supporting stacks of very carefully cut stone blocks.

I'd go instead for the enormous, late-Mediaeval,  Perpendicular-style cathedrals; built mainly in stone.

These are extremely daring constructions, with huge, soaring naves typically over 90 feet high to roofs of heavy timber frames covered with lead or slate. They show a very good understanding of how to use the arch as a structural support, without knowing the deeper mathematics involved. Sometimes a bit too daring though, as collapses of partly-built or even complete buildings were not unknown. (Usually by over-loading, but apparently, sometimes due to skimping on the foundations to cut costs... Nothing new!)  

Yet the masons and carpenters responsible had only hand-tools not very much more advanced than, probably, the Ancient Greeks or Romans used.   There is far less mystery here of course. Contemporary pictures give clues how these places were built. The masonry and woodwork preserve traces recognisable to a modern builder, of the techniques. The same modern mason or carpenter still uses essentially the same tools in the same way, for hand-work.

The real mystery is how the master-masons gained the collective skill and nerve even to consider erecting these magnificent glories-to-God, in the way they did; hundreds of years before anyone could invent 100-ton cranes and consultant project-managers. They make modern buildings of similar size, very tame by comparison.

Just learnt a new angle on Stonehenge, that its Neolithic builders did not have to move the stones very far at all.

The monument's main rock (and the name of the pillars) is Sarsen Stone, which is a sort of natural concrete apparently formed by interactions between river-gravels and chalk on which they have been deposited. This material in common around the Chalk downland on which Stonehenge stands. 

The trilithons though, are a different matter. These are of a rock called "blue-stone" (actually dolerite), whose nearest outcrop in Pembrokeshire, West Wales, is somewhere around 200 miles away by the modern roads and bridge across the Severn estuary. There are also other rocks, from that region, there too.

However, the latest edition (No.108) of the magazine Down To Earth, published for amateur geologists, reviews a book, The Stonehenge Bluestones, by Dr. Brian John, from Pembrokeshire himself and who gained his PhD by studying  the ice-age geology of the region.

By analysing what's known of the ice-cover and glacier flows some half a million years ago, Brian John calculated that the Welsh rocks used in Stonehenge were actually carried Eastwards and dumped on Salisbury Plain as "erratics" by the ice.

The Neolithic builders some hundreds of thousands of years later, didn't have to drag the rocks two hundred miles, by rollers, sleds and rafts, across a lot of hilly countryside and a wide, difficult estuary, after all. They found them possibly only a few miles from their building-site, and probably worked them into the rough rectangular blocks in-situ to make them slightly lighter and easier to move, before hauling them to Stonehenge!   

I think I will buy this book. The review is very favourable, and Stonehenge is a fascinating site.

Perhaps its real mystery is not how this thing was built. We can work that out fairly easily. Instead it is why it was built, for what purpose. We can only make intelligent but vague, general guesses about that without being able to prove it.