The Hills Have Eyes 2

A captive womanâforced to breed mutant childrenâgives birth to a stillborn. She is then killed by mutant leader Papa Hades (Michael Bailey Smith) for being unable to provide healthy offspring. Later, scientists working in the New Mexico desert are attacked by Letch.
A group of National Guardsmen in training are sent into the desert to resupply the scientists, who were working for the United States Department of Defense on installing a surveillance system (implied to be a result of the events in the first film). The soldiers arrive to find the camp apparently abandoned, and outside radio contact impossible due to the topography. When the radio operator, Spitter (Eric Edelstein), picks up a faint distress call, the sergeant (Flex Alexander) organizes a search and rescue mission, leaving behind Napoleon (Michael McMillian) and Amber (Jessica Stroup).
The search party discovers the mutilated body of a scientist in the hills, while Amber and Napoleon pull another dying scientist out of the portable toilet. On her way to join the group Amber is attacked by Stabber (Tyrell Kemlo), but a returning Mickey (Reshad Strik) wards him off. Just as Napoleon catches up, Mickey is pulled into a bolt-hole and killed. At the same time, the remaining troops are attacked by Letch, and the sergeant is accidentally killed by Spitter's friendly fire. Napoleon and Amber reunite with the group, and Spitter is killed by an unseen mutant sabotaging his rappelling gear as the others attempt to lower him down the hill. With their remaining gear stolen they are forced to try to find another way down.
The remaining troops soon locate their commanding officer, who has clearly become unhinged from recent events. He warns them of the mutants' plans to capture women for breeding and kill everyone else. He then commits suicide after telling them the only way down the hill is through the mining caves. After the group kills Stabber, Missy is captured and taken into the mining caves by Chameleon. Chameleon attempts to rape her, but Missy fights him off, only to be captured by Hades, who scares Chameleon away and savagely rapes Missy himself. The remaining troops attempt to rescue her, with the exception of Stump (Ben Crowley), who is killed by Letch while attempting to climb down the hill without ropes. After being separated from Crank (Jacob Vargas) and Delmar (Lee Thompson Young), Napoleon and Amber cross paths with Chameleon, who they kill. They later locate a non-violent mutant named Hansel (David Reynolds), while escaping Grabber (Gaspar Szabo). Grabber is killed by Crank after shooting Delmar. After reuniting with Napoleon and Amber, Delmar dies from his wounds, and Hansel leads Napoleon, Amber, and Crank to the exit. Along the way Crank is killed by a trapped crate of dynamite that he attempts to take with him, incidentally triggering an explosion.
After killing Letch, Napoleon and Amber find the captive Missy, and all three fight Hades, who they manage to kill as well. The three leave the mines, but are watched by an unknown mutant using their surveillance equipment.

Question: At the camp the outside radio contact was impossible due to what?
Answer: the topography