What's 1 mile wide, 300 miles long, and taking 14 hours to pass a single point?

Woolly Mamoths' tissue was preserved in ice, all over Siberia.

I'm not sure we have viable DNA for these pigeons.

To save everyone some time, this is a link to the wikipedia article for passenger pigeons, a once plentiful but now extinct species that inhabited north america in large numbers. The cryptic headline is a reference to a questionable sighting of a large flock of these bird potentially numbering 3.5 billion (billion with a b).