There are tens of thousands of immense mounds in North America alone.

One of them at Cahokia:

Monk´s Mound, Cahokia´s biggest, is 32 metres tall, spreads over 6.5 hectares and has 623,000 cubic metres of earth (!). A monastery was built on the top of it in the 1800´s, which is how it got the name.

[link to www.interlog.com]

There is no reasonable explanation, except of course my own...