Imagine coming back from a camping trip in the desert.  Then a few months later you notice in your photos from the trip that you camped inside a giant impact crater so big that no one documented it before.  Could it happen?
During several trips to the Black Rock Desert, mostly while supporting the Stratofox Aerospace Tracking Team for suborbital space rocket launch efforts, Ian Kluft KO6YQ noticed some oddities in rock formations.  He had a little experience with volcanoes, and some rocks in the area looked unusual.  There seemed to be some lava here and there - but where was the volcano?  He observed that even a large caldera should have mostly volcanic rocks.  He then noticed some curved geographic structures in satellite imagery which made him curious if it might be an impact crater.
A TV documentary about meteor impacts mentioned some characteristics of impact craters. He went to the Internet to learn more.  He noticed some of his own pictures of the area had cone shaped structures which might be "shatter cones" due to an impact shock wave that passed through the rocks.  Discussion with others produced suggestions and volunteers who joined the effort.  More circular features in satellite photographs were found.  Igneous dikes through white layers of rock had been described and were visible in many photos.
The possible impact crater is 30 miles (48 km) wide east-to-west and 40 miles (64 km) north-to-south centered around 40.984045 N, 118.916016 W.  That is in northwestern Nevada halfway between Reno and the Oregon border.  The apparent ancient geological structure is so old that much of it is eroded away.  The forces that eroded the Black Rock Desert itself, whether glacier or stream, have apparently cut through the crater rims and floor to do it.
Following an expedition to the area in late January, more possible shatter cones were observed in one of the rock samples collected.  These are only formed by the shock wave of an impact event or nuclear explosion.  The nearest nuclear test was underground at Sand Springs Range in central Nevada.  Atmospheric tests at the Nevada Test Site were at the southern, and opposite, end of the state.  So that points toward an impact if confirmed.  But professional geologists will want to have the final word on confirming them.
In addition to shatter cones, rock samples were thought to contain shocked quartz because the criss-cross fractures looked like examples in online documents.  But this group of volunteers doesn't have equipment for proper photography of shocked quartz.  If found, that would be another way to prove the impact origin of the rocks.
Columnar jointed columns hundreds of feet tall appear on some bluffs in locations that appeared consistent with part of a slowly cooling crater floor.  But that alone only helps as part of a bigger picture, because volcanoes can have lava cool in columnar joints as well.
They also found local geological studies which described oddities which could be explained by an impact event.  Layers of rocks in the mining district called Sulphur left geologists with a mystery about the cause of chemical alterations since 1980.  The group compared it with information in online geological texts like "Traces of Catastrophe" by  Dr Bevan M French of the Smithsonian Institution.  Layers of impact ejecta seemed to explain the rock layers better than the previous theory about acid uniformly cooking the rocks across the region, and only in one layer of rocks.  A separate 1980 study 40 miles away identified an immense air-fall tuff layer in the Soldier Meadows area as having been deposited in a single unit, yet couldn't locate the volcano which produced this enormous volume. Fault diagrams published online by a mining operation at Sulphur on the edge of the circular structure from the satellite photo also look like curved terraced faults in the wall of a crater.
The mining geologists who wrote papers from 1980 to 2002 had not mentioned the possibility of a crater.  But they were each gathering single puzzling pieces of information.  A larger image seemed to be forming when putting the pieces together.  The group hopes the information will be helpful to geologists who do further work in the region.
The theory hasn't been put to any test by professionals yet. Some responses point out that the elliptical region reported as the possible impact crater could also be the volcanic caldera that the 1980 study was looking for.  If so, such a large volcanic caldera would still be a significant discovery for the region.
This will remain officially a mystery for a while until the professional geologists get to study it. There is a lot of information available online for those who are interested.
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Rocketeers find possible impact crater in Nevada