Inaccessible means, with respect to a place -"unable to be reached". But I want a word or expression that signifies that a place is "almost inaccessible but can be reached ultimately with great effort"—like the North Pole or the hill area around Mount Everest. It is not like they can't be accessed at all; they can only be accessed with great difficulty. So what can I say these places are?
If you had asked for a word for the journey itself, rather than an adjective to describe such destinations, I would have suggested expedition, which is often used in historical contexts to describe arduous treks to hard-to-reach locations such as the Earth's poles, its highest mountains, or its deepest ocean floors.
I checked a couple of reverse dictionaries to see if I could find a word that would fit your request, but came up empty-handed.
Depending on the context of your description of such places, I might suggest saying something along these lines:
Maybe this simplest word is hardly. Collins dictionary defines it as the following:
scarcely; barely ⇒ we hardly knew the family;
just; only just ⇒ he could hardly hold the cup;
with difficulty or effort:
Use it like this:
— Can I reach Everest?
— Hardly. Everest is barely reachable if you have not trained well.
One may also consider hard to reach, depending on the connotation to convey.
There aren't any good, simple words that convey this meaning exactly to my knowledge, but there are some similar ones:
Inaccessible is perhaps the most obvious one, meaning that the place is difficult to reach.
Inhospitable is a similar word, meaning that the place is difficult to live at. For example:
Antarctica is the most inhospitable continent on Earth
You might want to add emphasisers to make your sentence more clear that the place is really inhospitable, for example:
We had to delay sending supplies to the Antarctica Scientific Outpost because after the weather declined because the entire place became practically inaccessible from the ground.
There are a couple of words I can think of for specific scenarios - for example a "redoubtable fortress" is one that is awe-inspiring and difficult or near-impossible to break through. "Forbidding" can also be used idiomatically to mean "making an approach difficult or impossible", as in "the forbidding terrain near the North Pole": http://www.merriam-webster.com/dictionary/forbidding