He called a friend from the local museum, and together they soon uncovered another 150 Viking relics. But the crops growing in the fields hindered their work and they gave up.
The following summer, with crops that year infected by lice, they resumed their search -- and on July 16, 1999, came across the biggest Viking-period treasure hoard so far discovered.
It had been lying there for about 1,100 years.
The Spillings hoard, described by archaeologists as a once-in-a-lifetime discovery, includes 14,295 silver coins, 486 silver armlets, and dozens of other artifacts, weighing a total of 85 kilograms.
"It was totally crazy," said Bjorn Engstrom, the farmer who owns the land. "I was there for five days when they dug up the treasure. I didn't leave the field," he said.
"The first night we camped there in a tent so nobody could come and take it."
Engstrom, 42, whose family has owned the land since only 1966, was not able to keep any of the silver himself.
Buried treasure was believed to be guarded by dragons in the days of old, but nowadays Sweden's law on historical monuments sets strict penalties for anyone searching for treasure with metal detectors, or failing to report any buried gold, silver or copper to the police or local museum.
Anyone discovering and dutifully reporting treasure gets a reward in line with the value of the metal. Engstrom is still waiting for his, as archaeologists have studied only a fraction of the Spillings hoard, named after his farm.
The complete hoard, including some bronze relics also discovered at the same site, will be on show in Stockholm's Museum of National Antiquities until Sept. 1 before returning to Gotland.
Archaeologists believe the treasure was buried in about 870.
The site appears to have been a farm even then, said Majvor Ostergren, project leader at the County Museum of Gotland.
"But this treasure is too big to be on a traditional farm," she said. "People there must have been something special."
The site's proximity to one of Gotland's main natural harbors, may be one clue.
As well as their fearsome reputation for plunder, the Vikings were also great traders.
They penetrated to Constantinople, now Istanbul, but then the capital of the Byzantine empire and one of the world's richest cities, providing soldiers for the emperor and trading with the Greek merchants.
The extent of their trading links was revealed in 1954 on the little island of Helgo near Stockholm, where a sixth century Buddha from northern India was found in a Viking site.
Although Gotland had few resources of its own, its position in the middle of the Baltic between Sweden and Latvia made it an ideal base for trade. The Vikings could bring in furs and amber from Scandinavia and the Baltic coast, and ship them along rivers into Germany or down to Constantinople.
That explains why of the 1,400 coins from the hoard that have been examined so far, four are Nordic, one from Byzantium, 23 are Persian, and the rest are Islamic.
The earliest coin in the hoard dates from 539 and is Persian, before the Islamic conquest. The latest is from 870.
One of the most important coins in the hoard, dating from 830 to 840, sheds light on a place far away: its markings show its provenance is the kingdom of the Khazars, a realm in southern Russia between the Black and Caspian seas.
Its Arabic inscription reads "Moses is the messenger of God" -- apparently a Jewish variant on the Islamic credo "Mohammed is the messenger of God".
Only four other coins are known to have this inscription.
The Khazars were believed to have converted to Judaism -- possibly the only nation to do so -- after their ruler invited Christian, Islamic and Jewish theologians to demonstrate the merits of their different faiths to his court.
Although many written sources describe the Khazars as Jews, few objects have been found in excavations in Russia to confirm these reports. The Khazar coin is thus important evidence.
But for visitors to the exhibition who are neither numismatists nor historians, the most fascinating exhibits are the hundreds of silver armlets.
The armlets are linked in bunches, indicating that they were used as money with a set weight, rather than jewelry.
Many are etched with a snakeskin pattern, and carry a serpent's head at one end. The striking simplicity of these austere but exotic bracelets suggests that the art of the Vikings lives on in modern Scandinavian design.