When the massive tsunami waves of the 2011 eastern Japan earthquake rolled back out to sea, they pulled with them fragments of docks, boats, and buoys that sometimes contained living stowaways. These hitchhikers—including crustaceans, mollusks, and fish—rode debris “rafts” through ocean currents over thousands of kilometers before finally washing ashore on Hawaiian and North American coastlines, sometimes years later. Now, researchers who studied hundreds of Japanese marine species living in and on this tsunami debris conclude that they were witnessing a “transoceanic biological rafting event with no known historical precedent,” they report today in Science. The scientists studied more than 600 pieces of tsunami-ejected debris—mostly nonbiodegradable fiberglass and polystyrene foam—that had swept up along Hawaiian, U.S. mainland, and Canadian shores. They counted nearly 300 Japanese species colonizing the debris, including roughly 80 different species on the largest piece, a 170-ton section of dock. In many cases, the scientists found that the animals had successfully reproduced along their journey. That’s evidence that the relatively slow movement of the debris (2-4 kilometers per hour) helped species adapt to changing conditions across the Pacific Ocean, the team wrote. But life hitching a ride between continents might have a dark side, the researchers noted: Debris could transport invasive species, forever changing ecosystems.