They said to each other, “Come, let’s make bricks and bake them thoroughly.” They used brick instead of stone, and tar for mortar. Then they said, “Come, let us build ourselves a city, with a tower that reaches to the heavens, so that we may make a name for ourselves and not be scattered over the face of the whole earth.” Genesis 11:3-4
Archaeologists have discovered ziggurats, ancient temples with broad bases and sloping sides that were created as sacrificial platforms. These massive structures do not technically "reach to the sky" but they show the ingenuity and construction know-how of ancient man.
Man had the power to build a very tall structure.
God was not scared by their ability to build a tower that would actually the heavens, that was impossible, but God was concerned about their reason for building a tower to "make a name for themselves." This nationalistic pride made them think they were more powerful than God.
God gave us creative skills since He made us in his image. Men could do incredible things if they set their minds to it. In this case, it made them selfish, prideful and dangerous, so God had to put a stop to it.
http://en.wikipedia.org/wiki/Ziggurat