January. February 51which are found only here, in the US state of
Florida, and on Mexico’s Yucatan Peninsula, all of which
all have similar karst limestone layers. Geologists are
still trying to understand precisely how the Old Home-
stead formed. The main theory is that groundwater ate
away at the Nullarbor’s bedrock, making it porous.
Then, during a wetter era, rainfall seeped into small
cracks, slowly dissolving the limestone and forming
an extensive network of passageways. Over time, pock-
ets near the surface collapsed, creating dramatic
entrances. “Most of the theories about caves have been
developed in Europe and North America, where run-
ning water has been the main process creating caves,”
Ian says. “In those places, limestone is generally com-
pressed, hard, raised up, and hilly – with rivers and
streams all carving away at it. Instead, our caves have
been dissolving quietly beneath the ground.”
Satellite imagery appears to support the hypoth-
esis that ancient groundwater had a role in creatingthe unusual cave systems. Although the Nullarbor is
largely stable, fractures in the rock suggest there may
be some flexing of the enormous layer of limestone
bedrock. It makes sense, Ian says. “It’s the world’s big-
gest slab of limestone, and it’s huge and thin. If that’s
the case, it is most likely that the longer caves are
forming along these cracks.”
This process might also explain Old Homestead’s
length. “Old Homestead is a bit of strange one.
It’s way out in the middle on its own, but it’s 34km
long,” says Ian, who also argues that its position at the
centre of the Nullarbor may be key – the centre is
weakest if the whole slab is flexed. Imagine pushing
at the edges of a soap bar – the middle is likely
to crack, letting in more water to expand and
lengthen the cracks.
For Graham, whose team returned in 2015 to map
unknown sections of the cave’s second level, it’s
a puzzle that still holds his attention. “There’s so much
of it and we always find something different,” he says.
“One day you walk this way and you’re looking in
one direction, the next day you’re looking in another,
and you find another passage and something
completely new.”Over time, pockets near the
surface collapsed, creating
dramatic entrances.
Using a photographic memory, Graham Pilkington
has created many of the maps of the Old Homestead
Cave by hand. He often works to add new details in the
old shed that sits near the entrance of the cave system.SEE more stunning pictures of the Nullarbor’s caves
online at: http://www.australiangeographic.com.au/issue130AG