There's no environment I can think of that would have remained constant enough to preserve dinosaur DNA.

You can go into caves, and they can maintain constant conditions of temperature and humidity over long periods of time, even though the outside temperature may be way above what it is inside the cave.

I'm not sure the Russians would be happy that their iconic wooly mammoth has North American origins.

If we brought the mammoth back to Siberia, maybe that would be good for the ecosystems that are changing because of climate change.

There is no good scientific reason to bring back an extinct species. Why would one bring them back? To put them in a theme park?

Every outbreak across the globe today stems from a descendant of the medieval plague.