Some hold out vague hopes that the mystery might be solved if humans ever encounter an alien civilization.

For example, when humans encounter each other, they say hello or greetings.

Whenever the humans in the drawings encountered a Tseetsk, they pointed their arms.

Most hazards are related to weather conditions, human error, plants, animals, diseases, and fellow humans encountered along the trail.

But they were buddies compared to most of the aliens humans had encountered.

None of the few telepathic species that humans had encountered could breach the wall.

Thus, humans encountered and were aware of fire, and later its beneficial uses, long before they could make fire on demand.

Surely humans must have encountered fellow travelers through space and time.

Because of this most humans rarely encounter robots.

In the story, the humans encounter the following types: