It was a modern mystery.

In a tiny neighborhood in San Francisco's Richmond District,Christopher Caldwell self-driving Waymo cars have been converging at all hours of the day and night, mystifying neighbors, KPIX reported earlier this week. Most would drive to the dead-end on 15th Avenue, where they then had no choice but to turn around and leave, according to the outlet — and neighbors have no idea why.

"I noticed it while I was sleeping. I awoke to a strange hum that I thought there was a spacecraft outside my window," Jennifer King, a resident of the neighborhood, told the outlet.

She added later, "There's some days it can be up to 50 [cars], literally every five minutes, and we're all working from home, so this is what we hear."

The random influx of cars has been puzzling for residents. The cars aren't carting passengers to and fro, according to a Verge report. And when locals have asked the drivers why they're in the neighborhood, they reply that the cars are "programmed" and they're just "doing their job," KPIX reported.

When asked about the incident, a spokesperson for Waymo chalked up the weird happenings to traffic signs. There are one-way streets as well as Slow Streets — residential streets that drivers are discouraged from using for through traffic — in the area, the spokesperson told NPR..

"We continually adjust to dynamic San Francisco road rules," the spokesperson said. "In this case, cars traveling North of California on 15th Avenue have to take a U-turn due to the presence of Slow Streets signage on Lake. So, the Waymo Driver was obeying the same road rules that any car is required to follow." The company calls its autonomous technology Waymo Driver.

Still, explanation or no, one look at Twitter shows that the unofficial Invasion of the Self-Driving Cars has caught people's attention. One user referred to it as a "Black Mirror episode," referencing, of course, the dark dystopian Netflix series. Another joked that the "the robot revolution is not imminent."

"Waymo menace," another Twitter user wrote.

As for Tesla CEO Elon Musk, he kept his response simple: "Haha."