According to Facts and Details, Marco Polo’s travels to Asia were, in part, fascinating for what he claimed to have heard while making his way through the Silk Road. “When a man is riding through this desert by night and for some reason — falling asleep or anything else — he gets separated from his companions and wants to rejoin them, he hears spirit voices talking to him as if they were his companions, sometimes even calling him by name,” he wrote in “The Travels of Marco Polo.” He also warned that these voices and strange noises sometimes had a negative effect, luring travelers away from their path as they ultimately got lost, never to be seen again.

Although it is indeed risky to stray from your path just because you hear something unusual in the middle of the night, Polo added that travelers heard these spirit voices in the daytime. As such, he pointed out that they would guard against the possibility of someone in their party getting lost or killed by traveling “very close together” and taking extra precautions at night. “Before they go to sleep they set up a sign pointing in the direction in which they have to travel, and round the necks of all their beasts they fasten little bells, so that by listening to the sound they may prevent them from straying off the path,” he explained.