URUMQI, May 22 (Xinhua) -- A wolf is at the door, literally, at a railway construction site in the Altun Mountains, northwest China's Xinjiang Uygur Autonomous Region.

The animal was first spotted sitting on a slope some 500 meters away from the construction site in the autumn of 2018.

"Some of us were shivering with fear at the sight of it. We didn't dare move for a while, then we started to back away very slowly," said Zhang Shaoshuai, a worker with China Railway Tunnel Group.

Zhang and his colleagues first encountered the predator on their way back to their dorm after work. "Some of the braver guys grabbed their cameras from the dorm and took photos," he said.

The wolf's arrival created quite a buzz among the workers. They discussed ways of defending against the animal, but they assumed that it would be gone by the next day.

However, the wolf has returned almost every day since. It roams and rests on the slope, sometimes pawing the ground in the daytime. At night, its eyes glow in the darkness.

The lone wolf never gets too close to the workers or steals food. But sometimes it sneaks into the yard and plays with the dogs. No more wolves have joined it so far.

Concerned for the safety of his workers, the foreman, Han Shun, called the forest police. Experts speculated that it might be a scout for a nearby pack, or it might have lost a fight and been driven away.

The forest police have suggested there's no need to take action just yet, as the animal doesn't seem to pose a threat to the workers.

In fact, the unlikely neighbors are getting along quite well. They can sometimes get as close as 50 meters. The workers don't taunt or bother the wolf, but the videos they've taken have received thousands of likes online.

The wolf is a Class B protected animal in China and hunting it is prohibited. There is a nature reserve covering 45,000 square kilometers of the Atlun Mountains, which are also home to endangered species such as the Tibetan wild yak.

Han thinks that the arrival of this wolf is a sign of ecological improvement in the area.

"Before the wolf came, we had also spotted lots of foxes and hares around here. This is their home," said Han.

A lot of effort has gone into environmental protection in Altun over the past few decades. Ecosystems have regenerated on the nature reserve since it was set up in the 1980s to keep poaching, illegal trespassing and mining at bay.

Some 400 km away from the uninhabited reserve, 3,000 meters above sea level, the workers are building a railway connecting cities in the northwestern provinces of Xinjiang and Qinghai.

To protect Altun's waterways, every day the workers descend from the mountains to fetch water from a town 100 kilometers away. They also build special passageways along the railway so animals can follow their normal migration routes unhindered.

But the wolf doesn't seem to be going anywhere anytime soon. The workers say it has "fallen in love" with one of their dogs, after noticing the two had started strolling around together.

"We're happy to let the wolf stick around," said one worker, "After all, we'll be gone once the railway is finished."