One of 2015’s weirder crime trends was the string of sometimes large rocks being thrown from various I-35 overpasses in Austin. The incidents haven’t been isolated: between June 2014 and the end of 2015, there’ve been more than 40 cars hit, seemingly at random, by someone dropping rocks onto the interstate. The first case of 2016 occurred early Sunday morning as a taxi traveling with three passengers—a married couple and an infant—was struck with a rock. As in previous cases, there’s no apparent motive.

There’s little known about the attacks on passing motorists. Police set up cameras on certain overpasses—they’re not saying which ones, for obvious reasons—but so far haven’t made any arrests related to rock throwing. (A man was arrested in 2014 following a report that he was throwing objects from an overpass, but appeared to have only been dropping plastic shopping bags.) It’s not clear if the attacks are perpetrated by the same person or if there’s a spate of rock throwers in Austin. There doesn’t appear to be a pattern to the attacks, and police can’t even confirm with 100 percent certainty that the rocks are coming from overpasses (though that certainly appears to be the most likely scenario) rather than from other cars.

Whatever the details, the consequences here are the same. Although none of the incidents have killed anyone, the injuries sustained by some of the people who’ve been targeted have been serious. Kenneth Johnson, who was injured last year near I-35 and Manor Road in Austin, suffered severe brain damage; as of November, he was still unable to talk and could not walk without a cane. (Another victim of an attack also suffered a critical head injury, and at least three others appear to have suffered lesser injuries.)

Arrests for this potentially dangerous crime were made in Waco in 2013—and in that case, the people arrested were kids, at least one of whom was as young as eleven-years old. It’s possible that the people dropping rocks in Austin are also young, which might explain why these attacks appear to be carried out by people without a fully-developed sense of consequence or empathy.

In any case, the amount of damage people are capable of doing to random strangers with no discernible motive—along with the sheer number of these attacks that have happened in the past year—makes this particular crime unique: It’s easy to do, can ruin (or end) lives, and serves no purpose. However it’s eventually addressed—whether by cameras or additional investigations to catch the perpetrators, or just by building fences to make it harder for rocks to come off of overpasses—it’s a weird epidemic that’s hard to understand.