The ruins' deteriorated condition requires urgent intervention as an empty chamber beneath Toompea tänav, which leads uphill to Toompea, threatens the street's stability.

Toompea tänav was already partially closed to traffic earlier this month as the surrounding earth began to subside.

If successful, the project may open the historic underground passages to the public, as with the nearby bastion passages.

The fortification is known as the Wismari Ravelin — a ravelin is a triangular fortification or detached outwork, located outside the inner works of a fortress.

The structure dates back to when northern Estonia was under Swedish rule, from the late 16th to early 18th centuries.

The tunnels, finished in the late 17th century, remained in use until the mid-19th century, during the Crimean War, when British and French naval attacks hit coastal settlements in Estonia, then part of the Russian Empire.

They were used again during the March 1944 Soviet bombing of Tallinn, when over 1,000 people took refuge there.

The burrowings still have potential as bomb shelters today. Their location became clear to city authorities in 2003, during the construction of a large apartment block across from St. Charles Church (Kaarli kirik), although it was sealed again after the work was finished.

A more thorough intervention from archaeologists is now needed to secure Toompea tänav and prevent collapse, as retaining walls have begun to crumble.

Ragnar Nurk, archaeologist at Tallinn's urban planning department, said: "At the moment, we have started with the surveying and investigative work. We will soon start with the geological drilling to determine exactly what the tunnels are supported by and how thick the vaults are, measured from the surface of Toompea tänav."

"It was located in front of Tallinn's main Swedish-era gate — at the place where Toompea tänav now ascends to Toompea. During the fortress period, there were wooden bridges here that crossed over this ravelin," Nurk added.

The tunnels are estimated to be a couple of hundred meters long, with one stretching to Hirvepark and the Danish Embassy.

King Charles XII (1697-1718) of Sweden had grand plans for Tallinn's fortifications, aiming to build 11 bastions, but the outbreak of the Great Northern War delayed progress, and only three — Ingeri, Rootsi, and Skoone — were completed. Along with these bastions, there were ravelins, earthworks, and tunnels beneath the bastions, though the tunnels were constructed at ground level, with earth piled over them.

As Toomas Abiline, senior curator at the Tallinn City Museum, said, "The idea was that beneath these bastions surrounding the whole city, there would be tunnels, so you could basically walk around the city inside them."

These tunnels, still stretching about 1.5 kilometers, prevented enemy miners from breaching the fortifications, and the ravelin allowed defenders to fire across the moat.

Postimees reported that heritage conservation work to save the structure means traffic restrictions in the area.

The chamber beneath Toompea Street is a vaulted space, five to six meters high, with a central column supporting the vaults. It housed cannons to fire across the moat.

While the chamber remains hazardous, the intervention work aims to prevent street collapse, with potential to open it to the public.

The tunnels were built to prevent the fortifications from being mined, with defenders positioned to counter-dig or intercept enemy tunnels, the report added.

Other tunnels in Tallinn, including bastion passages, are open to the public from the Kiek in de kök tower.

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