The wreck, of the HMS Cassandra, a light cruiser, is located in a zone, just West of Saaremaa (see map above), notorious for shipwrecks and unexploded ordinance, and from more than one major conflict. Indeed, it was a sea mine which sent the Cassandra to the bottom, not during World War One itself but in a related conflict, the Russian Civil War.

Since the fuel leak is very gradual, there is no significant pollution risk, however, the ministry says, as it dissipates once on the surface.

Kaspar Anderson, adviser at the marine environment department at the Ministry of Climate, said: "The leak is minor, and slow, so that fuel that does reach the surface tends to break down and disperses under the effects of both waves and sunlight."

"Only thin traces of dispersed oil can be observed on the surface," Anderson added. "The leak is not of the kind that had been previously assumed when detected via during satellite or aerial surveillance."

Nonetheless, the Cassandra could be holding in the region of hundreds of tonnes of fuel, and given the vessel's name, it might be worth not minimizing any potential warnings the gradual leak may be giving us.

"It is believed that there was up to 780 tonnes of fuel on board, some of which leached into the sea at the time of sinking. Considering the integrity and condition of the wreck, it is assumed that most of the fuel will still be inside the vessel," Kaspar Anderson added.

A total of 54 environmentally hazardous wrecks have been identified in Estonian waters to date. The environmental hazard of 13 of these wrecks has been examined in more detail, with more such studies likely to follow, the ministry says.

Making safe hazardous wrecks is in itself a very expensive and risky undertaking, meaning it is undertaken only if the situation can present a serious threat to the surrounding environment or inhabitants.

This means the wreck of the Cassandra is being monitored to determine if the leak grows in volume over time. The Ministry of Climate is also planning to initiate a cooperation project involving the relevant authorities, in order to remove the fuel from the wreck.

Laid down in 1916 at Barrow-in-Furness, then in Lancashire, England, the Cassandra's career was short lived. She was commissioned in June 1917, and sunk after striking a sea mine on December 5, 1918, with the loss of 11 lives.

Britain's Royal Navy Baltic Squadron provided aid to Estonia and Latvia in their wars of independence, 1918-1920, which were facets of the concurrent Russian Civil War.

The Royal Navy had transitioned from coal to oil as the main fuel source for its vessels in the years leading up to World War One.

The Cassandra's wreck was rediscovered in 2010, lying in around 100 meters of water.

The video below shows a recent wreck dive.

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