Remember! No one has proof that Atlantis really existed. Some people believe that Atlantis is a myth or legend, while others believe it really existed. Like any very old story, there may be some bits that have been made up over the years and other bits that are true. No one can be certain of the truth yet, but maybe one day evidence will be found and we'll find out for sure if the Atlanteans exist
Who might they have been? 
There was a Greek philosopher named Plato who told us the amazing story about the Atlanteans. His stories are our only source for knowing about these people. He said they lived somewhere on an island continent "Beyond the Pillars of Hercules". The Pillars of Hercules is an old name for the gap between Europe and Africa where the Atlantic Ocean joins the Mediterranean Sea. This would mean that Atlantis was somewhere in the Atlantic Ocean.
Plato said that the Atlanteans first walked into this world 11,000 years ago - that's a pretty long time ago! He also said that they were a very powerful, rich, noble race and that they were very clever scientists knowing things that nobody else did.
Some people think that the legend of the Atlanteans could have been based on the city of Minoans. Atlantis is a European myth so it is likely that the Atlantis stories were based on a European people. The Minoans lived near to Greece and volcanic activity in the area may have "destroyed" Minoan cities in historical times in a similar way to the destruction of Atlantis that Plato described.
Before Plato, even a simple bronze sword might have looked like cutting edge technology, so this may be where the part of the legend that says the Atlanteans were advanced came from. Minoan cities also saw a lot of technology from the Assyrians, Babylonians, and Egyptians because these people had to travel through the Minoan cities on their way to Greece. The knowledge and writing of these countries would have made the Minoans quite advanced in their learning as well. This theory seems to fit the legend well.
Which country they might lived in 
We do not know where Atlantis, the homeland of the Atlanteans was. There are many theories and speculations about its location and a lot of people from Europe and Africa claimed Atlantis was located in their land. What we know about Atlantis is that Plato described the Atlanteans' home as beyond the "Pillars of Hercules" (known as 'Straits of Gibraltar' now) on an island in the Atlantic Ocean. The Atlanteans may have lived on Thera an island off the coast of Greece.
What Plato said 
Plato was a Greek scholar, and also like a student crossed with a teacher. He said that...
"Atlantis was established by the sea god Poseidon. Poseidon had ten sons. Poseidon's eldest son became the ruler of Atlantis. Atlantis was divided into ten sections and each of Poseidon's sons got the kingship of one. The capital city, also known as the City of Atlantis, was renowned for its excellence in science and engineering. The city was circular and surrounded by land and water. At the center of the city was a high mountain called Olympus where there was a temple dedicated to the god of the sea, Poseidon, with a golden statue of him driving a six winged horse."
This was the way Plato described the city of Atlantis. According to this description some think Atlantis was somewhere near the Azores Islands and that the Azores are the leftovers of destroyed Atlantis. However there are an almost limitless number of places that people think it could be, some claim Cyprus is the remnants of Atlantis, while others claim it to be New Zealand, Tahiti , Cuba or one of many other possible locations.
The Legend 
The original legend of Atlantis says that the City of Atlantis sank beneath the sea. Many scientists and treasure hunters say that this is possible, but most people think this is just a story. Since 1960 many divers and submarines have looked for this ancient city but so far nobody has found it.