From the Spanish Antarctic research station Juan Carlos I. It's a seasonal base located on Livingston Island at the tip of the Antarctic Peninsula.
There are three postmarks on this cover. The first on the stamp was 01-01-18 at the base. Probably on or around the date the cover first arrived at the base. There is a more graphic Juan Carlos postmark on the left that is dated 24 May 2018. I'm guessing that's when the cover left the base. And finally the Barcelona postmark dated 08-10-18, when the cover was sent back to me from Spain.