jm ÊQ éWóô'ɲ}¢!Tá=_ðFÂA6èN´3L£I am well aware that there was migration to the Americas before the land bridge migration. This link doesnt claim there were massive cities 20000 years ago though.
https://whatsonchain.com/tx/818d4b6759d7844582c00c281e6bf72563d430a1bb5cc33cf2bd84fe320fea1d