Spear-fishing pearl spots for lunch has rarely been more lucrative.

On March 20, Nishant, a local youth, free-dived into the Sasthamcotta freshwater lake with a metal-tipped wooden spear to lance a few “Karimeen.” Summer often causes the lake to recede, and spear-fishing is a seasonal hobby for the young men.

Nishant sensed a “shimmer” on the lake bed and thrust his spear at it. He felt its tip strike something metallic, which turned out to be a copper vessel containing 146 copper coins.

Archaeology Department Director Reji Kumar said Nishant had accidentally stumbled upon the largest “treasure hoard” of ancient Chinese coins to be discovered in Kerala so far.

In 1988, State archaeologists had recorded the finding of two ancient Chinese coins on the shore of the lake. More recently, they discovered four ancient Chinese coins from a historical site at Kottapuram in Thrissur.

He said the department would seek the help of the Archaeological Survey of India, which has a marine excavation wing, to scan the lakebed for more submerged ancient artefacts.

R. Rajesh Kumar, Research Assistant, Numismatics, of the department said the copper coins had Chinese inscriptions and symbols only on one side, indicating that they could have been minted in the early 12 century. Some coins had a “Pi” symbol on their “blank” side.

Local divers who searched the lakebed near the spot where the coins were discovered came up with more copper vessels, lamps and a grain measure, which sports the alligator symbol of the 13 century royal house of Veera Kerala Varma, the king of erstwhile Venad.

State archaeologists said the ancient port of Kollam, similar to Muziris (Pattanam) in Kodungalloor in Thrissur, had shared trade relations with the Chinese, Phoenicians and Romans.

They said the etymology of several place names in Kollam, such as Sambrani Kodi and Chanku (ancient names for Chinese vessels) and Chinnakada (Chinese trading post), revealed a strong Chinese link. Experts said there was geographical evidence to show that Sasthamcotta Lake was once linked to the Ashtamudi lagoon and the spot where the coins were discovered could have been part of a now lost inland waterway for country boats that brought rice, pepper, dried fish and betel nuts to the ancient port.

Another theory was that some trader or feudal lord could have hid the treasure underwater to prevent them falling into the hands of marauding forces of the early 18th century king Marthanda Varma when he subjugated the kingdoms of Kollam and Kayamkulam in the 1730s.