hello and welcome

i have a script below to understand the exact meaning if i use simple past or present perfect

"It was after midnight in a remote part of the western desert. A team of archaeologists were asleep in thier tents in a wide flat valley with high hills of sand on both sides.

At the top of one hill, two grey Land Cruisers began to move slowly down towards the tents. The two cars moved without lights and thier engines were silent.

they stopped 200 meters from the tents. Four men got out. Although the cars had local number plates, the men were not Americans. They walked 50 meters through the darkness and then seemed to dissappear into a wall of rock.

they had climbed down into a long underground passage that was two meters wide and two meters high."

that script i got it from a story but i am confusing what is the difference in meaning if the writer uses " they climbed down" and "They had climbed down" ? why he used past perfect tens ? i need the exact meaning if i used one of the two tenses