You've heard of Stonehenge, the Neolithic monument near Amesbury, England. It's made of stone blocks that were probably set up as a kind of astronomical calendar.
No one is quite sure what the social function of the stones may originally have been, but here's something that will surprise you: stonehenge isn't alone.
Researchers exploring the Amazon have discovered a series of prehistoric stone blocks that have been intentionally arranged to point in different directions. That's interesting enough, but on December 21st the shadow of one of the blocks disappears entirely. That suggests that the block was angled specifically to produce this effect, which makes researchers think the whole thing may be an ancient astronomical calendar.
Anthropologists are surprised at this finding, because it suggests a more advanced ancient culture lived in the Amazon than had been understood. Whoever erected the blocks was already asking questions about the sun, the earth, and the cycle of the seasons the kind of questions today we call astronomy.