A cenote is a natural phenomenon, a sinkhole in the Earth’s surface. The Yucatan Peninsula in Mexico has an estimated 7,000 cenotes because it is primarily made up of porous limestone. For millions of years, rainfall slowly ate away at the limestone and a huge system of underground caves and caverns was formed. Many filled with water from rain or from the underground water table. When the roof of a water filled cave collapses, a cenote is born. The water found in a cenote may be fresh water, salt water, or both. Structurally it may be completely open, like a lake, almost completely closed with just a small opening at the top, or somewhere in between.

An activist kicks the shields of the military police officers during a demonstration in the military zone of the 27th infantry battalion in Iguala, Guerrero, January 12, 2015. Activists and relatives of the 43 missing trainee teachers from Ayotzinapa's teacher training college broke into the military zone, located less than a mile from where the students went missing, in an attempt to look for the missing students.The remains of only one of the 43 students has been identified so far. (Photo by Jorge Dan Lopez/Reuters)

The twin lighthouses of St Joseph on Lake Michigan have been transformed by a winter storm. Ice encases the buildings, giant icicles hang from the piers and the beach is frosted white. Here: Huge icicles hang from the pier. (Photo by Mike Kline/Barcroft Media)

As you look at the majestic Crater Lake, located in south-central Oregon, United States, it is hard to imagine that thousands of years ago this place was one giant crater with scorching hot lava spilling over the sides. Those were the times of turmoil. Now, however, this place is the essence of tranquility. The lake that was formed, after the volcano died down, is amazingly clear due to the fact that the water isn’t moving at all. No rivers go in or out of the Crater Lake, and the water lost through evaporation is compensated by the rain and snowfall. This lake is also frighteningly deep – 1,943 feet (592 m) at its deepest point – making it the deepest lake in the United States. If you love nature and if you ever come to Oregon, this is truly a sight to behold.

Kelimutu is a volcano, close to the small town of Moni about 50 km to the east of Ende, Indonesia in central Flores Island of Indonesia.