IPA: /ˈmaʊntənəsnəs/
KK: /ˈmaʊntənəsnəs/
The condition of having many mountains or being full of mountains.
The mountainousness of the region makes it a popular destination for hikers and climbers.
Mountainousness → It is formed from "mountain" (from Old French *montaine*, meaning a large natural elevation of the earth's surface) and "-ous" (meaning full of) and "-ness" (meaning the state or quality of). The word "mountainousness" refers to the state of being full of mountains or having many mountains.
Think of a place that is 'full of' ('-ous') 'mountains' — that's what mountainousness means.