Every 26 seconds, the Earth gives a faint tremor. It sounds improbable, and it has been recorded for sixty years without anyone being able to explain it.
The pulse — a microseism — was first documented in the early 1960s by Jack Oliver, a geologist at Columbia University’s Lamont-Doherty Geological Observatory. He did not have the instruments a modern seismologist would use. Later work narrowed the source to a place called the Bight of Bonny, in the Gulf of Guinea off West Africa.
The two explanations
The shelf as a drum. Some researchers think the cause is mundane. The continental shelf beneath the ocean acts as an enormous wave barrier, and ocean waves striking a particular part of the shelf in the Bight of Bonny at a regular rhythm could produce the pulse. On this reading the shelf’s structure resonates much as a drum does, producing a particular note.
A volcano. Others think a volcano is responsible. The centre of the pulse sits very close to a volcano on the island of São Tomé in the Bight of Bonny, and volcanic seismic pulses of this kind have already been documented in Japan.
Why it has not been solved
This is the genuinely interesting part, and it is about science funding rather than about the Earth.
Scientists have studied the pulse, but because it carries no risk of major natural damage, solving it has never become a priority. Nobody dies of it, no insurer prices it, no government is exposed to it. Sixty years of unanswered question is therefore not a measure of how hard the problem is; it is a measure of how research effort gets allocated — towards the questions somebody needs answered.
There is a comparable seasonal oddity on the record: the Earth’s seismic vibration tends to fall every year in the run-up to Christmas. The cause there is not mysterious at all — industrial activity and traffic quieten — and it is a reminder of how much of what seismometers pick up is us.
What a seismometer is really listening to
Worth knowing if you follow earthquake news in a country that has reason to. A seismometer sensitive enough to record a 26-second pulse from West Africa is also recording ocean swell, wind on mountains, passing lorries and construction.
Separating a real earthquake signal from that background is most of the discipline, and it is why early automated alerts are sometimes wrong and why a network of stations beats a single instrument. For Bangladesh, sitting on an active and under-instrumented seismic zone, the number of stations is the thing that determines how fast and how accurately a real event is identified.
How you locate something you cannot explain
The pulse was traced to the Gulf of Guinea without anyone knowing what makes it, and the method is worth knowing because it is how every earthquake is located too.
Seismic waves travel at known speeds through known rock. A station records the arrival time of a wave; a second station, far away, records it slightly later. The difference narrows the source to a curve, a third station narrows it to a point, and more stations tighten the estimate. None of it requires understanding the cause — only accurate clocks and enough instruments.
That is why the 26-second pulse could be pinned to a stretch of West African coast decades before anyone proposed a mechanism, and it is a useful reminder that in seismology locating and explaining are entirely separate problems.
