The Colosseum was just a stadium. A pretty one, but really just a big building among other big buildings. The great pyramids were the effort of a whole nation. They're the Delta Works, the ISS, the Panama canal, the Three Gorges dam. The Colosseum the equivalent of Michigan stadium...
You commented some really cool things, and then mentioned the 34th biggest sports venue (3rd largest stadium).what's significant about Michigan stadium?
The point is: Michigan stadium is not on the same level as those other constructs. This Facebook user is doing the equivalent of "It took 5 years for my city to approve permits and build a new library. Extrapolating from that, it should have taken 400 years to build the Hoover dam. Aliens must have done it"