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The sun sets on a field in southern England. When it rises again the following morning, that field has been changed into a huge work of art. A large part of the crop (庄稼) has been pressed into a pattern of circles, squares and other shapes.

Farmers have reported finding strange circles in their fields for centuries. The earliest mention of a crop circle dates back to the 1500s. Crop circles, however, didn’t get attention until 1980, when a farmer in England discovered three circles in his field. The world first began to learn about crop circles.

By the 1990s, crop circles had become something of a tourist attraction. In 1990 alone, more than 500 circles appeared in Europe. Within the next few years, there were thousands.

Crop circles can come in many different shapes, changing in size. Most early crop circles were simple. But after 1990, the circles changed. Crops can be made to look like just about anything — smiling faces, flowers or even words.

People have long been wondering who or what is creating these crop circles. Possibly the most widely known idea is that crop circles are the work of visitors from other planets.

Probably the most scientific opinion says that crop circles are created by vortices (涡旋). They force the air down to the ground, which presses the crops down. Dr. Terence Meaden, an expert, says when the dust (沙尘) gets caught up in a vortex, it can appear to glow (发光), which may explain the UFO-like glowing lights many people have seen.