Terra Incognita

"...animal pathogens remain a scientific terra incognita."

http://www.nytimes.com/2014/10/14/science/rats-and-their-alarming-bugs.html

Terra = land
Incognita is essentially incognito which means more or less hidden.

If something is hidden you essentially don't know where it is so it is unknown.

If you search for "terra incognita" it means land unknown.

There is also the land Tierra del fuego = spanish for "land of fire".

Often in sci-fi and fantasy you hear the word tera or terra or something like that and it refers to earth.

It is interesting that when I have run into the word land is almost always referred to in the feminine form with the A at the end.  It makes complete sense because it is from the land we are able to survive just as we depend on our mothers for nourishment when we are born.