While I was in San Blas (a little town just south of Mazatlan in northwestern Mexico, I saw a HUGE grass and Moss covered pyramid. The locales told me it was an Aztec pyramid, but the Mexican government had fenced it off and filled it with poisonous snakes to keep people away.
Apparantly they did not want people to know of this anomalous location of an Aztec pyramid. It suggests that the Aztecs moved so far north, they may have even broken through into northern america!
Anyway, my point is, sometimes there are cover ups of anomalous finds because they don't follow popular belief, or they may induce a 'political' reassessment.
EDIT - I didn't mean to suggest I believed there were egyptian ruins in the grand canyon. Because I don't.