The South Pole has had a permanent residential scientific community since 1957. Its official name is the Amundsen-Scott South Pole Station, and it is owned, and operated by the United States. A fleet of LC-130s maintain a daily cargo service during the summer from October to February flying in from Christchurch New Zealand. The population varies from over 200 in the summer to under 90 in the winter when it endures six months of night, and no flights come in or go out.
Because of the great expense of getting people into this site and maintaining them there it isn’t a viable site for 1000 people on a permanent Lifehaven basis. Even fuel costs 20 times per gallon what it does back in New Zealand, where it is brought in from. They are absolutely dependent on the outside world for food supplies, and energy for heating and electricity. All of Antarctica suffers from those extreme limitations although those on the periphery or on Palmer Pennusila are cheaper, and might be viable.
The South Pole Station would be a perfect place for an unattended seed bank. It has a very low average temperature and rarely gets above zero degrees F., and because of the high altitude it has low atmospheric pressure. In this location seeds would never germinate, and would age very slowly. Recorded temperature has varied between a high of −13.6 °C (7.52 °F) and a low of −82.8 °C (−117 °F). The annual mean temperature is −49 °C, which means that the ice a few meters below the surface would be constant at very near that temperature. Therefore, all that is needed for a seed bank at this location is to place a well made barrel with selected seeds a meter or more below the surface and mark it in such a way that it can be found for a very long time into the future. The buildings at this location are constantly sinking into the ice and are constructed upon pilings so they can be jacked up occasionally and stay above the surface.
If they were constructed in the form of barges they would float up in the water. It is frozen water, of course, but there is still a hydrodynamic pressure difference between the bottom, and the surface creating buoyancy just as there is in liquid water. It is probably too much trouble to construct a pressure resistant barge beneath the buildings, but the same principle of buoyancy applies to our smaller seed barrels. They would float rather like buoys at sea but they would have seeds packed into them. The cheapest way to do this is to have a loaded heavy barrel at the bottom, denser than ice, with a solid shaft connected to an empty barrel at the top, much less dense than ice, and a flag pole sticking up from that which would remain visible above the surface so long as the barrels floated. More elaborate but better would be to construct a vertical cylindrical barrel 3 meters in diameter and 30 meters deep heaver, and stronger at the bottom to resist the hydrostatic pressure, but adjusted to be buoyant in ice. It would have a spiral staircase from top to bottom and the walls would be filled with drawers filled with seeds. Either, or both of these structures should remain stable, floating, visible and the seeds viable for thousands of years.
For people to recover the seeds a very long time into the future might be as difficult as it was for Scott who walked into the South Pole in 1909, and died of starvation trying to walk out. However, at some distant time in the future it might be worth the effort and the risk to come to this location. But those intrepid survivors would have to know exactly where they were going, and what to expect when they got there. What they find can never be more than what we put there so we should choose carefully. What they take back to their world will probably prove of incalculable value and permit the planet to be terraformed back from a scorched cinder into something a little more like the beautiful one we presently live in, and are in the process of destroying.