Slaves. Take the Romans for example. They shipped huge pieces of marble and set them strait to build the HUGE buildings that still stand today. I forgot the auctual numbers but tons upond tons. How? Not Aliens. They found other methods such as building a ship Around the marble, shipping it off, then uprightening it using other means.Think about it, how did the Egyptians build it in a small amount of time then? :o
And Atlantis is a fairy tale o.o
"Aliens" is a very overused term that has lost meaning in my opinion.
"a creature from outer space; extraterrestrial." Does not mean they are more intellegent than us.
I'm not argueing that they don't exist. In fact, it's very possible as seen in the example of earth. However, it's unlikely that they are like us, or anything that can process a thought. More like Bacteria.
As for the paintings and carvings, think of how UNLIKELY it is for other creatures to evolve JUST like humans. It's nigh impossible that they have similar arms, legs, body shape, etc. And Helicopter? Flying things? How about today? Space Time Warp, Light Sabers xD In the same sence, these things seemed far away and were great for imagination. It's great for Sci Fi though :)