Depending on whom you ask, those mysterious field carvings known as crop circles are: communications from outer space; by-products of the earth’s energy; or Led Zeppelin fans, paying large-scale homage to the band’s greatest-hits collection. Even The Sixth Sense director M. Night Shyamalan has some thoughts on the matter; his new movie, Signs, looks at how a Pennsylvania farmer (Mel Gibson, pictured at right) deals with ground markings in his backyard. Where better to explore the confluence of science, conspiracy theories, and alien encounters than on the Web? After a thorough online investigation, we found a few sites that offer a dash of mystery, a bit of artistry, and some old-fashioned fakery.

SIGNS ( Don’t look for answers here: The film’s official site offers nothing about the secret Gibson uncovers at the movie’s end. It does offer a handy primer on the past 300 years of crop-circle history. In one case, says an expert quoted in the ”What Do We Know?” section, witnesses reported seeing football field-size formations rapidly materialize…before their very eyes. Spooky. A-

CIRCLEMAKERS (circle In 1991, Doug Bower and Dave Chorley confessed to a 13-year spree of circle-making in which they employed a decidedly low-tech method: a rope attached to an iron stake. Their tradition is carried on by Circlemakers, a group of artists who design and create elaborate formations under cover of night. They’ve even devised a circlemaking program (downloadable on Macs) so you can design your own in either wheat or barley. A

CROPCIRCLES.CO.UK ( Signs may be set in the farmland of Pennsylvania’s Bucks County, but most circle activity occurs across the pond: Of the 184 crop formations reported in 2001, 102 were in Great Britain. Here you’ll find detailed maps and photos of U.K. circles going back to 1996. But last time we checked, the site’s ”crop shop” was closed. In July. The high season of crop-circle activity. Hmmm. B-

CROP CIRCLE QUEST ( Photographer Judy Arndt has recorded crop circles all over Canada, and her images — from dazzling close-ups to panoramic vistas — show a rare perspective on the markings and their scale. The site also has links to reports by the BLT — a team of research scientists who found meteoric iron deposits at one formation in England — and The Crop Circular, which publishes intricate theories on Euclidian crop geometry. But where, we ask, can we find an explanation for the strange formations on David Letterman’s head? B+