If you see a shark fin slicing the surface of the Río San Juan, you will be forgiven for thinking you’ve come down with heatstroke. In fact, there are sharks in this freshwater river!

The creatures, along with other species normally associated with saltwater, migrate between the Atlantic Ocean and the murky waters of Lake Nicaragua, navigating 169 kilometers of river and rapids en route. The sharks are classified as euryhaline species—they can cross from saltwater to freshwater and back again with no ill effects.

For centuries, scientists were confounded by the sharks’ presence in Lake Nicaragua. The lake is separated from the Pacific by a 17-kilometer chunk of land, and, since rapids on the Río San Juan seemingly prevent large fish from passing easily from the Caribbean, surely, the thinking went, the lake must have once been connected to one or the other ocean. Uplift of the Central American isthmus must have trapped the sharks in the lake.

Studies in the early 1960s, however, showed that there were no marine sediments on the lake bottom. Thus, the lake was never part of the Atlantic or the Pacific. (It was actually formed when a huge block of land dropped between two fault lines; the depression then filled with water.)

Then, ichthyologists decided to tag sharks with electronic tracking devices. It wasn’t long before sharks tagged in the Caribbean turned up in Lake Nicaragua, and vice versa. Incredibly, the sharks are indeed able to negotiate the rapids and move between lake and sea.