Between the reefs in the central Pacific and the lonely Clipperton Atoll to the east lies a huge expanse of water. It was once thought that coral larvae could not cross it, because the currents mostly carry water the other way. But geneticists noticed that corals on Clipperton and on the Line Islands to the west are very similar. It is as if there is a secret road between them.

That road was found using drifting buoys. For thirty years they floated across the ocean, and scientists recorded their paths. It turned out that in summer and autumn a rare current appears that can carry a coral larva from the Line Islands straight to Clipperton. The journey takes about two and a half months, and a coral larva can live up to five months, so it has time to arrive and settle in a new place.

Now it is clear why Clipperton Atoll has become a genetic bridge. It is not isolated but connected to distant reefs by rare seasonal currents. This matters for coral conservation: even far-flung places can depend on one another.