While we do a great deal of exploring above ground - in forests, mountains, and even in our atmosphere - generally we haven't explored as much below ground.
These holes in the Earth were created in a variety of ways - some man-made, and some naturally-formed, but all of them look like portals into another world. Here are some of the strangest, and deepest holes in the Earth!
1) Glory Hole, Monticello Dam, California USA
2) Mount Baldy Sink Holes, Indiana, US
This man-made hole is the spillway of the Monticello Dam in Napa, California. At the highest lake water point, the spillway can drain 48,400 cubic feet per second of water (swimming near the glory hole is obviously prohibited).
These sink holes started to pop up all around the southern portion of Lake Michigan beginning in 2010. One unlucky kid even got trapped in one in 2011 (he was fortunately saved and was safe). While this one is only 11 feet deep, many others can grow to twice that depth or larger.
3) Kola Superdeep Borehole
A research project of the Soviet Union, the Kola Superdeep Borehole was opened in May of 1970. The project studied biological and geological features of the subsurface material of the earth - researchers even found microscopic fossils that were dated at nearly 2 billion years old. Unfortunately the drilling stopped after the temperatures became nearly 2 times what was expected near the bottom of the hold.
The first photo shows the borehole itself, which is now welded shut. The second photo shows the drill tower that housed the machinery.
4) Ice Cube Neutrino Observatory
This unique observatory is based in Antarctica, and is comprised of 86 separate holes drilled into the Antarctica ice up to 1.5 miles deep. Together, the sensors at the bottom of these holes provide researchers valuable information about high-energy particles throughout the Universe. The holes were drilled in the summertime using massive drills and hot water.
5) Great Blue Hole, Belize
This hole was naturally made, and it is one of the best scuba diving locations in the world. Being so close to land means that there are tourists and divers visiting all the time.
6) Dean's Blue Hole, Long Island, Bahamas
Similar to the above deep blue hole, this hole is near land in the Bahamas and is also a wonderful tourist destination. This particular hole is the deepest saltwater blue hole in the world - nearly 700 feet deep.
7) Guatemala City Sinkhole
At 100 feet deep, this strange sinkhole suddenly swallowed a 3-story building in 2010.
8) Kimberley Mine, South Africa
This massive diamond mine has produced more than 6,000 pounds of diamonds. The massive hole started as a small hill, and over many years more than 50,000 miners dug out diamonds mostly by hand. It has since filled in with water.
9) Derweze, Turkmenistan
Called the 'Door To Hell', this hellish burning hole has been on fire since 1971. It sits above a natural gas deposit in Turkmenistan, and has been on fire constantly since Soviet workers ignited it while drilling for gas.