Photo credit National Park Service
On the 12th July, 2013, six-year-old Nathan vanished into the Mount Baldy Sand Dune off the shores of Lake Michigan.
When Nathan was dug out three hours after he was swallowed, Mount Baldy was found to be filled with subsurface caverns and holes, which current theories render impossible.
Sand dunes form when wind blows sand into obstacles - such as vegetation, opposing winds or denser air - and the sand is deposited. Over time, windblown sand particles are stacked atop of one another and a dune forms.
Unlike quicksand or sinkholes, there isn’t any room for caverns or hollows to form.
One idea is that since Mount Baldy isn’t anchored down by vegetation, it can ‘wander’. Between 1938 and 2007, the dune had moved over 130 metres inland! This quick movement buried things beneath the dune, and the wet conditions in 2013 may have caused them to collapse, forming these caverns. Nevertheless, science is still stumped.
And that’s just one of the many examples of weird stuff we’ve found in the rocks and landscapes of planet Earth.
This is different from quicksand which is when sand is saturated with water that can’t flow away. This non-compressible water is trapped in the sand, taking the weight of anything on top of it. These waterlogged sands will behave like liquids and cause things to sink.
Two Billion Year Old Nuclear Reactor
Though we’ve only had working nuclear reactors since the 1950’s, in 1972 French workers at the Oklo Uranium mine in Gabon, Central Africa, found something weird.
Uranium comes in three different forms (or isotopes), Uranium-234, 235 and 238. 235 is the isotope which can sustain a nuclear reaction and is also the rarest of the three. The ratios of the three isotopes should be the same everywhere on Earth. So when 200kg of Uranium-235 had gone missing (enough to make six nuclear bombs), scientists were left scratching their heads.
What they had stumbled on was the leftovers of a nuclear reactor which spontaneously began around two billion years ago, and produced a constant power output of roughly 100kw.
A natural nuclear reactor needs similar conditions to the ones we use to power cities: atoms which split easily, neutrons to split them, and a way to control the reaction. Two billion years ago, Uranium-235 concentrations were around 3% (about the same as modern day nuclear power stations use). The natural decay of the uranium into thorium provided the neutrons.
When Oklo was filled with groundwater, the neutrons were slowed down enough for fission to occur. But when the energy released from the reactions boiled some of the water away, the neutrons were no longer slowed down enough to allow fission. This stopped the reaction until new water percolated down into the aquifer below.
This cyclical groundwater flow (much like a geyser) helped to control the fission process and allowed the reactor to run for about a million years, before the concentrations got too low, and Oklo’s natural reactor turned off.
Eye of the Sahara
Photo credit Landsat 7 NASA
At 50km across, the Eye of the Sahara in Mauritania is so big, early NASA missions used it as a landmark. This almost perfect circle’s origins are still debated, but we suspect the landmark is actually a deeply eroded geological dome.
Around a hundred million years ago, the movement of the African continent caused the crust to weaken. Swelling subsurface magma pushed this dome upwards, forming what’s called an anticline; a large fold of rock which sticks up from the surroundings. This pushed the youngest rocks to the edges, keeping the older rocks at the centre. Hydrothermal circulation (water heated by the mantle around the anticline) likely helped to erode the dome.
Because the eye is made up of different rocks, they eroded at different speeds. Softer sedimentary rocks eroded faster than the harder igneous and metamorphic rocks (like quartzite), forming the weird concentric rings of the eye.
Giant Crystal Cave
In Naica, Mexico, a mining company began to drain a cave 300m below the surface looking for lead or silver. Instead, they found giant 12m high crystals - the largest ever found.
These huge crystals of selenite (a form of the mineral, gypsum) sit under Naica Mountain, which formed roughly 26 million years ago. The molten lava below the mountain brought anhydrite (dehydrated gypsum) with it.
At high temperatures, anhydrite is stable. But below 58°C, anhydrite will dissolve in water and reform as gypsum. Therefore, as the magma cooled, the anhydrite slowly dissolved and reformed as gypsum, which crystallised into selenite. Giant Crystal Cave was deep enough to stay at 58°C for over half a million years, which allowed the crystals to just keep growing.
What’s great for crystals, however, isn’t great for people. The cave is so warm, and the air is so humid (100% at times), that the surface of your lungs would be the coolest things inside it. This is why you’re only allowed in there unprotected for ten minutes - before the water vapour in the humid air condenses inside your lungs and you drown!
Door to Hell
Photo credit Tormod Sandtorv
Okay, this one is cheating a little because the Darvaza Gas Crater (aka. the Door to Hell) isn’t really a natural geological phenomenon.
In 1971, a group of Soviet geoscientists were exploring the Karakum Desert in Turkmenistan for oil and natural gas fields.
While there isn’t an official story behind it, it’s believed that a natural gas filled cavern collapsed after they began to drill over it, producing a twenty metre deep, sixty metre wide crater.
This venting methane posed a number of problems, namely in the form of the greenhouse effect (methane is twenty five times more potent than CO2), and that methane is flammable in air in concentrations less than five per cent.
The geoscientists could either allow the methane to leak into the environment and put local populations at risk ...or they could set it on fire.
44 years later, the crater is still on fire.