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Last Updated on May 5, 2015, by eNotes Editorial. Word Count: 431

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Wilson, a geologist, recalls his role twenty years earlier as leader of a lunar expedition to a massive plateau, the Mare Crisium, or Sea of Crisis. Although the name is portentous, initially the journey appears to be mundane. Wilson even laments that there is nothing hazardous or especially exciting about lunar exploration. It is, he claims, an uneventful routine.

Wilson’s expedition is well equipped. Traveling from the main lunar base, some five hundred miles away, the crew is laden with heavy equipment, including overland tractors, rockets, and scientific sampling machinery. It appears they have little to fear from the unknown. They are in constant radio contact with their base and can survive for a month in their pressurized tractor cabins if there were an emergency.

The men live in relative comfort during their tour of duty on the moon. Short wave radio contact with earth provides ubiquitous music for the men as they dine on freshly cooked food. This particular expedition, however, soon deviates from the routine. While preparing his breakfast sausages, Wilson observes a glint of what appears to be a metal object on a far mountain bordering the plateau. Against unanimous dissent from his crew, Wilson and his assistant, Garnett, journey to investigate the object.

After scaling a ten-thousand-foot-high mountain at the edge of the plateau, Wilson discovers a small crystalline pyramid. He initially believes that it was created by an extinct, previously undiscovered, indigenous culture. The absence of any other artifacts, coupled with the presence of a sophisticated force-field surrounding the object, soon prove to him that the artifact’s provenance is both extralunar and extraterrestrial.

Wilson describes the twenty-year process by which scientists seek to dismantle the artifact and ascertain its nature and function. When all methods prove ineffectual, the scientists resort to atomic energy, reducing the inscrutable object with its mysterious energy source to rubble. Its purpose never determined, the artifact, which had withstood natural destructive forces for millennia, is rendered inoperable by humans.

Wilson provides his interpretation of the significance of his discovery. The object, he believes, is a sentinel, one of millions planted throughout the universe by an unimaginably advanced race in order to watch over the promise of life. Wilson believes its destruction would signal its creators that sentient life had evolved from terran primordial soup, and had proved its fitness to survive by having ventured forth from its earthen cradle. Because the object’s destruction will alert its creators to the probability of human intervention, humanity, according to Wilson, surely will not have long to wait for its first extraterrestrial contact.