Every 26 seconds, the Earth shakes. Not a lot — just enough that seismologists on multiple continents get a measurable little “blip” on their detectors. And even though this pulse has been observed for decades, researchers don’t agree on what’s causing it. The pulse was first documented in the early 1960s by a researcher named Jack Oliver. He figured out that the pulse was coming from somewhere “in the southern or equatorial Atlantic Ocean” and that it was stronger in the Northern Hemisphere’s summer months. Many seismologists suspect it is caused by ocean waves impacting the continental shelf in the Gulf of Guinea, with the interaction creating rhythmic vibrations that propagate as seismic pulses throughout the planet’s crust. Others propose a volcanic cause, citing the proximity of the pulse’s source to a volcano on the island of São Tomé