Beaulieu was leading three French ships through Table Bay, decades before the Dutch would establish a permanent settlement at the Cape. In 1620, the bay was a waypoint -- a place where ships stopped for fresh water and provisions before continuing to the East Indies or returning to Europe. Robben Island, a low-lying landmass visible from the anchorage, was already known to European mariners. The seas were calm that April morning, the ships becalmed in the still water, when the two sharp reports broke the silence. Beaulieu, an experienced sailor accustomed to the sounds of the sea and the sky, found them noteworthy enough to record. His description -- two sounds like cannon shots, without visible cause -- became the earliest documented seismic event in South African history.

For much of the historical record, Beaulieu's account was accepted at face value. The event was catalogued as an earthquake of Mercalli intensity II to IV -- weak to light -- making it the oldest recorded seismic event on the southern African subcontinent. South Africa sits on a relatively stable tectonic plate, and earthquakes here are infrequent and generally mild compared to those along plate boundaries. The 1620 event, if genuine, would establish that seismic activity near the Cape predated European settlement by decades and suggested that the region was not as geologically quiet as its stable continental setting might imply.

In 2012, Sharad Master of the University of the Witwatersrand published a paper in the South African Journal of Science challenging the event's classification. Master examined Beaulieu's original account and concluded that the two thunderclaps were very likely atmospheric phenomena rather than seismic activity. Becalmed ships in subtropical waters can experience localized atmospheric pressure events that produce sharp, cannon-like sounds without any ground movement. If Master is right, the title of South Africa's oldest verified earthquake shifts to a slight tremor off Cape Town in 1690, which registered a Mercalli intensity of III. The distinction matters less for its seismological significance than for what it reveals about the difficulty of interpreting four-hundred-year-old observations made by people who lacked the instruments and vocabulary to distinguish between phenomena we now understand as entirely different.

Robben Island would go on to acquire far heavier historical weight -- as a place of banishment, a leper colony, and ultimately the prison where Nelson Mandela spent eighteen of his twenty-seven years of incarceration. But in 1620 it was simply a landmark in a bay, a reference point for navigators. Beaulieu sailed on, and his logbook entry gathered dust for centuries before seismologists noticed it. Whether the thunderclaps were the earth moving or the atmosphere cracking, the account preserves a vivid moment: a fleet of wooden ships, still on a glassy sea, and a sound that had no explanation. Sometimes the oldest mysteries are the simplest.