"inaccessible terrain" is correct and usable in written English.
You can use it to describe a piece of land that is difficult or impossible to access, such as a rocky mountain. For example: The mountain peak was surrounded by inaccessible terrain, making a summit attempt impossible.
Some eighteen million acres of mountainous and inaccessible terrain were declared off limits to development.
Active fire behavior in very steep, inaccessible terrain makes preparation of contingency lines critical.
Rugged, inaccessible terrain — and occasional drug cartel violence — complicate the conventional practice of backpacking in the equipment for electro-shock sampling of remote water bodies.
Along its course between Myanmar and Laos, the Mekong drains about 8,000 square miles (21,000 square km) of territory in Myanmar, comprising rough and relatively inaccessible terrain.
But the fire was still only 15 percent contained, she said, and because of winds, dry conditions and inaccessible terrain, it had great potential to move east and northeast, farther into and around Yosemite.
He added: "We had information from the outset that the objective of Linda's captors was to pass her further up the Taliban command chain and perhaps move her to even more inaccessible terrain.
The battlefields of the 21st century are almost certain to be in countries without large standing armies, air forces or navies, where overwhelming force will be less essential than swiftness and maneuverability and where austere conditions or inaccessible, rugged terrain will require self-sustaining units like those the Marines operate.
But already, there are signs that this war will end up being fought on the ground, where Special Operations forces, marines and some Army infantry have been joining the hunt for Taliban and Al Qaeda fighters across rugged, inaccessible, cave-riddled terrain.
The weevils will also disperse to other areas, including inaccessible or difficult terrain.
Pathfinder's primary mission is to test a new way of getting instruments to Mars that can land just about anywhere--including rocky, hostile terrain inaccessible to Viking-like landers, and the most promising niches for life.
The persistence of plague in Himachal Pradesh is favored by its hilly terrain, inaccessible area, inclement weather (snow) in winter.
