Gene Barrels in Antarctica are the easy solution to the long term survival of humanity because after the Doomsday wars there will need to be a source for recovery of the genetic heritage of billions of years of evolution of life on Earth. It is only in Antarctica where we can be certain of a thousand year storage of seeds and it is upon the food that these seeds will create that humanity will thrive.
With the destruction of many if not most of the food producing crops in the currently habitable world at Doomsday the surviving humans will have a pitifully small number of foods from which to choose. It is very difficult to know but people might be reduced to eating what we now consider to be slime. The problem becomes how to get large quantities of every possible gene into a safe and very cold storage place where they can survive totally untended by humans for thousands of years. With global warming certain to continue for at least a hundred years, most if not all glaciers and other icy spots outside of Antarctica will melt. Therefore even carefully tended seed-banks such as the one at a bomb proof facility called the Svalbard International Seed Vault will probably prove futile in the long run. In this case the long run may be as short as one hundred years, but certainly if things proceed as they presently are, one thousand years is an impossibility for that site.
A thousand years sounds like a long time but the thing under consideration is humanity which has a hundred thousand year long life history and even civilization has several thousand years to look back upon. Thus a thousand year projection is reasonable and the only reliable seed bank site is the two kilometer thick ice sheet in central Antarctica. At first glance this seems a formidable obstacle to reach but it can be done.