In the Genesis account, Noah built an altar after leaving the Ark and reaching dry land.
Genesis 8:20 says that Noah built an altar to the Lord after the Flood. He then took some clean animals and clean birds and offered burnt offerings on it. This is the first altar explicitly attributed to Noah in the Genesis narrative and marks his response after the long confinement aboard the ark.
The sequence is important: Noah does not first build a city, temple, or tower. The altar appears immediately after he leaves the ark, before the covenant sign of the rainbow in the following chapter. The offering is also connected with the earlier distinction between clean and unclean animals.
Later biblical stories contain many altars and elaborate sanctuaries, but Noah’s altar is a simple early act of worship. Confusing it with the Tower of Babel is especially easy because both episodes occur in Genesis, though Babel appears later and involves a different group and purpose.