The Great Ocean Road is one of the most popular tourist attractions in Australia. Within this dramatic coastline resides the Twelve Apostles, rocky pillars that jut out from the unforgiving sea. Around 20 million years in the making, these spires apparently have been hiding a secret for some time.
As a new study in the Journal of Coastal Research reveals, five more pillars exist nearby, but it’s unlikely tourists will get to see them: They’re located 50 meters (164 feet) underwater. Unsurprisingly, these new columns are being referred to as the Drowned Apostles.
Despite originally being part of the same limestone cliff, the new 7-meter-high (23-foot-high) stacks are 6 kilometers (3.7 miles) away from the others. Like their more easily viewed cousins, these concealed pillars are made of limestone, an easily erodible rock type.
Tens of millions of years ago, the limestone cliffs along the Great Ocean Road began to be eroded by the action of the waves. Caves formed when the waves began to bore right into the base of the cliffs, and after a few more million years, these caves were somewhat sizable.
The computer-generated sonar images of the five drowned stacks. Rhiannon Bezore
At this point, the thin arches at the top of each of the caves collapsed, leaving the seaward-side wall standing on its own; this is known in geology as a stack. When the stack is chipped away by the hydraulic action of the waves until there’s only a bit left sticking out of the sea, it is known as a stump.
These five new limestone stacks, discovered during a high-resolution sonar survey in the area, are therefore quite odd. “Sea stacks are always eroding… so it is hugely surprising that any could be preserved at that depth of water,” said David Kennedy, an associate professor of coastal geomorphology from the University of Melbourne, in a statement. “They should have collapsed and eroded as the sea level rose.”
These five submerged columns – the first of their kind that researchers know of – would have formed in much the same way as those sticking out of the water. However, it’s unclear at present how they’ve managed to survive the 60,000 years they’ve been around for. One possibility is that the sea level rose so quickly at the end of the last ice age 20,000 to 10,000 years ago that it simply rose above them; this meant that powerful surface waves couldn’t spend time eroding them.
Either way, it’s unlikely that the Twelve Apostles will become the Seventeen Apostles. For one thing, there are actually only eight of them visible, ranging from 30 to 67 meters (98.5 to 220 feet) high. There were nine to begin with, but one of them suddenly collapsed into the ocean in 2005.
Local authorities are adamant that there are in fact only seven true apostles still standing. With these new five drowned stacks, perhaps there really are twelve apostles after all.
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