I will first give you my general opinion about who built the megalithic sites all over the world. Following that you will find a picture gallery with examples.
I have long been interested in ancient civilizations, and one thing has become increasingly clear to me: before 10,000 BC there was a worldwide civilization with a certain amount of technology. The problem is that most people try to compare it to our modern civilization. I think this is wrong. I think it was a civilization which was very different than ours. Debunkers tell us that if there was a technological society we would have found proof of it, in the form of technological remnants of buildings, machinery and their technology. Well, the proof is all around us, but it is not recognized because they are looking with the wrong glasses.
From my own study it is clear to me that before 10,000 BC man had achieved a high level of civilization. But how do I define 'a high level'? Usually Atlantis is seen as a group of islands that once existed in the Atlantic ocean. This is where the main center was, but Atlantis stretched also out to part of the Atlantean continent, especially South America, and had many colonies all over the world. The power, the government, the technology was mainly in the Atlantean Islands. From their they dominated the world, building colonies, establishing new civilizations, helping the local people with technology. The way technology was used is different than what we do now. The Atlantean scientists had some technology that was being developed underground, that is literally under the ground. They tunneled into the ground and rock and built their underground laboratories well protected. It is no different than what is done today. All over the world, the military has been building bases and laboratories underground for at least a century. So did the Atlanteans. That is why we did not find remains of their technology, because archaeologists are only looking at the surface of the earth. These Atlantean tunnels, even entire underground cities, are present on every continent. This is not exactly a secret nor is it recent news. Ancient underground tunnels have always been known to exist, although few have dared to enter them, as these tunnels go on for miles and miles, even hundreds of miles. Apparently the Atlanteans must have had an automated means of transport to travel through some of these tunnels with high velocity. Present-day governments and military have always been aware of these tunnels and underground cities and have explored them when possible in the hope of finding some ancient technology. When we consider that for example Edgar Cayce said that the Atlantean civilization stretched from roughly a hundred thousand years ago to 12,000 years ago, then we realize that this is a lot of time during which they were able to develop their own technology, be it on a very limited scale. Not like today, where technology is being spread immediately all over the globe (although certain technology is being kept secret). In ancient times, technology was kept to the scientists and hidden away, and thus progressed slowly.
So Atlantean scientists had some technology, but were not sharing it with the general population. Sorry, no car or cell-phone for everybody. They didn't have profit-making commercial companies in those days. Originally they used their knowledge and technology to help people, but later on, as power corrupts, the Atlantean colonies used the technology to war among themselves and eventually caused the collapse of the entire Atlantean civilization. The last destruction of Atlantis happened very suddenly, as all over the world we see that the the activity in building megalithic sites came abruptly to a halt. In my article Wéris, a megalithic center in Belgium I have visited several times, one can still see the remains of cut and half-cut megalithic stones scattered around the quarry as if the workers one day just stopped working and walked away. We find those sites all over the world. We also find unfinished megalithic buildings. There too, the workers suddenly left their work unfinished.
When archaeologists first studied the megalithic stones in Europe, they put up the theory that they were built by an unknown Mediterranean culture that traveled around the Mediterranean and even up the North Sea to England to erect the big stones. It was said that they were only a couple of thousand years old, as they found human remains buried with some of the megaliths. But now that we are able to share information quickly with people all over the world, we find the same megalithic monuments on every continent. There are megaliths in the Soviet union, in China, in Africa, in North and South America. It is becoming obvious that it was not primitive stone men who put up these megaliths, but a sophisticated worldwide civilization. And they date from the time of Atlantis which is at least 12,000 years ago. The human remains they found are from later civilizations, who probably buried their leaders on these sacred sites, by way of last respect.
Stone cannot be carbon dated, so we do not know exactly when they were carved, but there are telltale signs that whoever made them possessed great knowledge of astronomy, mathematics, technology, earth energies, and healing. Stone circles, for example, are not always circular, they are sometimes elliptical. Certain megalithic sites have clearly been built with astronomical purpose. The most obvious feature is the lining up of stones to mark equinoxes and solstices. Why do local people need to know when the equinoxes and solstices are happening? The most obvious explanation is that this is crucial for establishing and maintaining a calendar. A calendar is absolutely necessary for agriculture. A year is not exactly 365 days, and even when you add an extra day every four years, eventually, the calendar will be out of sync.
The next purpose megalithic sites have is that some of them, if not most, tap into the earth's energy lines (ley-lines, dragon lines, and others). There are a couple of things you can do with the earth's subtle energy. First there is healing. Some of the stones have a reputation to heal certain diseases. Although I would not go that far, I have met enough people to know that some stones strongly emit energies that, when touched, flow through the body and can promote healing. When I was in Scotland, visiting megalithic sites, there were a couple of people in my group who could clearly feel the pulsing energy on a particular raised spot on one of the stones in a stone circle. In Avebury there is also a stone with two raised spots where one can put both hands to feel the energy. In my article about Sillbury Hill I explain how my sore throat was cured in only two hours. That convinced me that Sillbury Hill was another complex built by the Atlanteans to mark a major energy spot. Sillbury Hill, like the other megalithic sites, shows clear signs of architectural knowledge and planning. Another thing one can do with megalithic sites which are tapping into the earth's energy fields, is to raise the fertility of the land, again an important aspect for an agricultural society. Other megalithic sites, especially dolmens, were made for initiations. Some of them, more complex, have antechambers, where the initiate was being prepared. Some of the dolmen have a small circular opening in the roof, through which the initiate could focus and through which he would astrally project, causing out-of-the-body-experiences. The earth's energy spots upon which these dolmen were built would enhance this practice.
How where the megalithic sites built? I strongly believe that this was not done by hand alone, but by technology. There are plenty of legends that tell of the ancient builders using magical means, and even sound to cut, melt, shape and transport these heavy, sometimes, multi-ton, stones. Those devices were only used by the guild of Atlantean scientists, who took them wherever they went, all over the globe, and they were then taken back to their underground laboratories. So, there weren't that many devices around, and ordinary people had no access to them. It was a technology that we do not have, but that does not mean that it was more 'advanced' that ours. It was different. They knew how to work with stone, that is, with a small device they could cut stone, shape stone, and lift stone. Probably it used principles of microwaves, or high intensity sound waves, with which they also could cause anti-gravity. You don't need a super crane, bulldozer, or lift fork if you can make a stone weightless by a small device emitting a laser-like ray.
It is obvious that these devices existed, because why would man slave away in erecting multi-ton stones when life is already hard enough just to make a living. Would you do it? I have also found examples of really hard and difficult stones that were polished. Why polish a hard stone by hand? Unless you can do it my machine. I found two stones in Wéris  (the Devil's Bed and the "Allee Couverte Sud") that were polished; both are puddingstone. Puddingstone is a conglomerate rock and extremely hard. It is composed of little stones, sandstone, limestone, quarts and flint.
We should not only look to megalithic stones or stone circles, but also to the remains of Atlantean cities. Yes, remains of Atlantean buildings are around us, but archaeologists refuse to recognize them as such. I am speaking of those ancient buildings that have been built with equally gigantic stone blocks, beyond the means of primitive man. Some examples are given below. Often those cities have been occupied by later civilizations, like the Incas. Some archaeologists still claim that Machu Pichu was built by the Incas, while it is now known that the ruins of Machu Pichu were already there when the Incas arrived. Teotihuacan is another site of which the builders are unknown, and which shows clear signs of highly developed architectural and astronomical knowledge.
We are finding ever more ancient cities, some of them from the time of Atlantis, just under the water at coastlines all over the world. Some 12,000 years ago the water level of the ocean rose and covered all the ancient cities on coastlines. We know that most of the present-day population lives at the coastlines, so we may assume that it was the same in Atlantean days. Yonaguni, at Japan, although still disputed, is an example. remains of underground cities have recently been found in the North Sea, off the coast of India, of the coast of Cuba, and lets not forget off the coast of Florida where divers have seen remains of buildings through the ever shifting sands. I also remember a story of some ruins off the coast of South America. When I was in my teens, I read a small news article that said that a Russian ship, while taking pictures of the ocean floor in the Atlantic ocean, had taken some pictures of remains of buildings. One would think that it would make headlines, but I never heard of it anymore. Discoveries of Atlantic remains, are being made all the time, but because its existence is still not accepted, most of these discoveries do not make it further than the local news and there is a failure to connect it to Atlantis.

go to the Atlantean photo gallery.