Islands And Planets

Once, unexplored islands and even undiscovered continents played the same role in fiction as other planets. There was a time before real world explorers had penetrated north or south of cerain latitudes. The destination of Lemuel Gulliver's fourth voyage could be described as (not the Planet of the Apes but) the Island of the Horses. See image.

There were also the depths of the oceans, the centre of the Earth, darkest Africa and the upper atmosphere. In a Conan Doyle story, an aviator finds himself flying above one of the deadly denizens of the upper atmosphere...

Retro-sf restored the habitable Venus and Mars:

The Sky People by SM Stirling;
In The Courts Of The Crimson Kings by SM Stirling;
Old Mars, Eds., George RR Martin & Gardner Dozois -

- so maybe it can go back further to the unexplored Earth? Needless to say, our main man, Poul Anderson, would have revelled in such an environment.