They could be hiding in Jabal-Al Halal -- a rugged mountain terrain in central Sinai.

CNN: STORY HIGHLIGHTS

The mechanical "dog" is already able to move about a varied, rugged terrain while carrying a load.

BBC: BigDog four-legged robot now sports throwing arm

Yesterday, DARPA announced that an LS3 protoype has successfully completed several exercises, including the ability to follow a person using its sensors over rugged terrain outside.

FORBES: DARPA Robot Carries Heavy Loads So Soldiers Don't Have To

The practical difficulties of the North Korea's rugged terrain have in the past made efforts to build a traditional landline telephone network time consuming and expensive.

BBC: Cheaper calls for North Korea

Washington County is a terrain of gentle hills and close valleys that grows more rugged as you get east over toward the Vermont border, where the Green Mountains begin in earnest.

NPR: Excerpt: 'World Made By Hand'

The country's rugged terrain and poor road network also slows down overland journeys, whereas a helicopter can fly anywhere, at speed, with flexibility.

BBC: Helicopters 'are no magic wand'

In this rugged terrain, smoldering embers can shoot up to 8-foot flames in a matter of seconds.

CNN: Wildfire ravages home but spares family roosters