Until late last week it had been nearly 70 years since the last confirmed sighting of bottlenose dolphins in the Baltic Sea.
Three of the extremely rare visitors thrilled both locals and scientists by appearing off the coast of Kimitoön, a seaside municipality in southwest Finland.
On Tuesday morning, researchers set out underwater listening devices in an effort to monitor the dolphins' activities and were soon rewarded with their first acoustic observations.
Whale researcher Olli Loisa, a senior advisor in water and environmental studies at the Turku University of Applied Sciences, says that the audio data will help keep track of their movements.
"It's also possible to determine when they are feeding. There is plenty of food for them in the area," Loisa told Yle.
The listening devices have been installed in a bay where the dolphins have been observed on several occasions.
"The distance between the closest and farthest device is around five kilometres. We didn't go any farther out than the area in which sightings have been confirmed," Loisa explains.
Loisa says it is impossible to predict how long the dolphins may stay in the waters off Kimitoön.
He says they might spend a long time in and around Finland's southwestern archipelago, or continue their travels just suddenly as they appeared.