'Like snowshoeing on balloons made of yogurt -- only messier'

It is a strange sight, some 30 hearty souls, their heads lowered, sloshing their way ankle-deep through thick mud, bent for the horizon and a barren island five miles and two hours away.

This is wadlopenm (literally, walking on the Wad,m or tide flat), a Dutch invention, which according to the organizers of the sport, the Wadloop Centrum, located in this tiny town 200 miles northeast of Amsterdam, some 25,000 mud- lovers do every year.

It is done at low tide from May through September -- quickly. Occasionally, it means walking in water waist- deep, more often in mere mud, and if done responsibly (that is, with proper clothing, supplies, and in the company of a guide specially trained and equipped for such excursions), it is safe. Naturally, rumors abound -- one describes a huge Egyptian simply sinking out of sight; another finds several accountants off on a holiday losing their way in the fog and having to swim for their lives.

The Wadloop Centrum organizes group walks costing $5 to $10 round trip to six different islands strung 5 to 10 miles offshore. Some trips, such as those to Simonszan and Rottumeroog -- both 10 miles away -- are wadloped one way and made by boat on the return. Others are walked in both directions, with the fast-moving tide rolling in behind the scurrying mud-sloggers.

Legend has it that the Romans were the first outsiders to discover wadlopen,m reaching this part of the world on conquering missions to find the indigenous population living on man-made hills for protection from the sea. Twice in every 24 hours, the sea would recede, allowing formerly isolated families to reach shore. Why the people chose not to live inland like everyone else has never been explained.

To walk in the mud today for some reason other than survival is something else again. Exactly what it is is unclear, although, enthusiastic guides often educate wadlopersm on land-reclamation projects en route, or point out birds that come to the wadm by the thousands at low tide in search of food, seals on sandbars and other plant and animal life.

The Centrum advises would-be mud walkers to wear a knit cap, sweater, windbreaker, shorts, wool socks, and basketball shoes, which collectively may make the difference between the participant, if not turning up in the pages of Vogue, returning to normal life or to the sea. The organizers prohibit alcoholic beverages on the trips, request that walkers take along a lunch (packed in watertight bags), and suggest that cameras be brought along. The guides -- all volunteers -- are furnished with first aid kits and walkie-talkies that link the groups to the Centrum in Pieterburen. Children under 14 must be accompanied by an adult.

As for a reward for successfully completing one's first wadlop, the Centrum goes one better than leaving its charges with the mere intangible joy of what one veteran has described as "something like snowshoeing on balloons made of yogurt, only messier." It awards to everyone completing the task a signed diploma -- "as proof of accomplishment," it says. The Wadloop Centrum, Hoofstraat 118, Pieterburen, The Netherlands, Telephone 05952-345.