Canadian author Carl Borgen who joined the dig in 1987 has detailed his involvement in his new book, Temporarily Insane.
The excavations are on the family property of tour guide and mystic, Ior Bock. The unorthodox character, Bock was able through sheer conviction, folklore and cult of personality to convince a collection of rag-tag treasure hunters to follow him to Finland.
Having first learned about the treasure in Goa, from Finnish expats, he joined the excavation to see for himself.
Digging in the cave networks on the Bock's family estate, it is an annual undertaking that has drawn treasure hunters from all over the world.
Each summer the excavations continue a little further into the network and - possibly - closer to the riches.
Talking to The Mirror the author said he "spoke to them only last week and it is now their strong belief that, after more than 34 years of digging, they are now within metres of the temple entrance."
🔥Lemminkäisen temppeli. Edesmennyt opas, shamaani ja joogi Ior Bock tarinoi 1980-luvulla sukunsa mailla Sipoossa...
Bock was murdered in 2010 after a dispute with two assistants. Stabbed at his home in Munkkiniemi, SK24 reported that the motive was unknown.
Still, each summer after pumping 1.5 million litres of water out of the waterlogged tunnel system, the digs continue to this day.
"I understand that significant progress at the temple has been made and that the crew are feeling especially excited about the months ahead," Borgen told The Mirror.
"There is now talk in the camp of being on the brink of a major breakthrough, which in real terms could be the discovery of the world's largest and most valuable treasure trove."
Not all middle-age burial mounds are so tough to excavate, for so little reward.
On Wednesday, a Norfolk treasure hoard was declared to be the largest discovery of its kind. Over 130 gold coins had been extracted from the Anglo-saxon burial mound over the past 30 years.
The treasure dated back 1400 years was mostly uncovered by a single, anonymous metal detectorist between 1991 and 2020.
However, they may not be able to keep the loot. Under the 1996 Treasure Act any find of precious metals unclaimed for more than 300 years are defined as property of the Crown.
In 2017 policeman David Cockle was jailed for 16 months for trying to sell coins from the hoard illegally, reported the Guardian.
The Sutton Hoo regional park is famous for burial mounds and was recently the setting for 2021 Netflix drama The Dig, starring Ralph Fiennes.