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.

The combination of Venezuela's sky-rocketing prices and chronic product shortages have left many struggling to put regular food on their tables and maintain a balanced diet. According to one recent study, 87 percent of Venezuelans say their income is now insufficient to purchase their food needs. Here: All the food available in the house of Antonia Torres and her family, is pictured at their home in Caracas, Venezuela April 22, 2016. (Photo by Carlos Garcia Rawlins/Reuters)

Girls practice sit-ups during gymnastics lessons at the Shanghai Yangpu Youth Amateur Athletic School in Shanghai, China, on May 4, 2016. China's sports system has been enormously successful since the country returned to the Olympic fold in 1980, culminating with the host nation topping the medals' table at the 2008 Beijing Olympics. And yet, with the Rio de Janeiro Games less than three months away, the system is beginning to break down due to the shifting demographics of a more prosperous nation. (Photo by Aly Song/Reuters)

Chinese conglomerate Tiens Group employers take photos while waiting to be served Paella in Madrid, Spain, Friday, May 6, 2016. The billionaire founder of Chinese conglomerate Tiens Group is treating 3,000 of his best salespeople to a traditional Spanish paella meal in a free Madrid trip that also includes a bullfight and a tour of King Felipe VI's Royal Palace. The smiling salespeople washed down their heaping plates of paella with sangria Friday at a mass spread of picnic tables in a riverside park during the event footed by Li Jinyuan and organized by China's U Tour travel company. (Photo by Paul White/AP Photo)

Ornamental chicken breeding clubs have emerged in Indonesia, Thailand, North America and even European countries such as the UK and France. Malaysia is however the epicenter of this cultural phenomenon. These chickens are prized for their build, size, behavior and showmanship by their owners and competitions or beauty contests as they are often described are held almost every week in at least one village in Malaysia. Judges sit around a square table inspecting each chicken for a few minutes trying to determine a champion specimen in its own weight class based on its stance, temperament and physical assets like wings, tails and comb. The walk or strut by an ornamental chicken in a beauty contest, much like a runway model, constitutes a large part of the scoring system. (Photo by Ernest Goh/2013 Sony World Photography Awards)

A truffle is seen on a table in Alba, north-western Italy November 11, 2013. Located in the heart of the Langhe – the hilly southern area of Italy's northwestern Piedmont region – Alba is the country's capital of white truffles, a variety of the prized fungus which grows underground. Truffles are found two to eight inches (5-20cm) below the ground near the roots of trees. They give off an odour which lasts for a limited period of time and can be detected with the assistance of well-trained dogs and experienced hunters. Output of white truffles, which are not cultivated and only grow naturally in forests, has fallen in Italy over the past few years, largely because climate change has brought a damaging mix of drought and torrential rains. (Photo by Stefano Rellandini/Reuters)

Everyone wants to get something for nothing - whether it's a lucky upgrade that means you can turn left when you get on a plane instead or heading off into the economy seats, or even getting a few extra features thrown in for free when you buy a new car.

As some of the sharpest businesses around, no-one understands this quite as well as casinos and that's why they all have a system that they call comps. It's short for "complimentary offers" and these are special treats specially designed to reward you for your loyalty to the casino in question. They're basically bonuses.