Crop circles. Not many take them seriously, yet nobody has satisfactorily explained their occurrence – let alone the large, intricate one spotted by an aerial photographer in Salinas, California.

This specimen, near Monterey, isn't much like other ones found through the decades, which tend to be simpler. It does however have the hallmark perfectly-round circumference, and absence of credit. Also, the one seen on Monday is highly complex, perhaps befitting today's technological age. It bears noting, perhaps, that not all crop circles are round. There have even been triangular ones.

The first record of a crop circle – a rather simple one - is from the 1600s. However, the phenomenon only really took off after 1971.

A particularly vulnerable spot seems to be the south of England, which is either quite the fave vacation spot for aliens, or somebody's family tradition includes sneaking off in the dead of night with a quiet tractor.

Even Israel has known its crop circles, the first being reported in 1997, in Tel Yitzhak.

Crop circles are areas of flattened grass, corn or whatever is being grown in that field, in very specific geometric shapes. They can be huge: the biggest on record is more than a half-kilometer in diameter. While some are confident the vast marks on the ground are made by aliens, somehow secretly and silently visiting in the dead of night, over the years various people have confessed to creating them as a hoax.

Or there could be another culprit. In 2009, the Tasmanian attorney general accused wallabies of creating crop circles in fields of medicinal opium poppies. The hapless marsupials had eaten the potent flowers and started hopping in circles, he said.