Factslap: The Earth shakes every 26 seconds, and scientists aren’t sure why.

Like a lot of strange happenings, it was first noticed in the 1960s: A small seismic pulse, large enough to register on seismological instruments but small enough to go otherwise unnoticed, occurring every 26 seconds. Jack Oliver, a researcher at the Lamont-Doherty Geological Observatory, first documented the “microseism” and sussed out that it was emanating from somewhere “in the southern or equatorial Atlantic Ocean.” Not until 2005 was it determined that the pulse’s true origin was in the Gulf of Guinea, just off Africa’s western coast, but to this day scientists still don’t know something just as important: why it’s happening in the first place.

There are theories, of course, ranging from volcanic activity to waves, but still no consensus. There does happen to be a volcano on the island of São Tomé in the Gulf of Guinea near the pulse’s origin point, not to mention another microseism linked to the volcano Mount Aso in Japan, which has made that particular explanation more popular in recent years. Though there’s no way of knowing when (or even if) we’ll learn the why of this phenomenon, one thing’s for sure: better a microseism than a macroseism.