BERLIN — A whale that was stranded for days in shallow water at a Baltic Sea resort in Germany has swum free from a sandbank overnight after a last-ditch rescue effort. But it isn’t out of danger yet.

An excavator spent Thursday digging an escape channel. The whale then swam through it overnight, marine biologist Robert Marc Lehmann said Friday, German news agency dpa reported.

But he cautioned that it was only a small step in the right direction for the marine mammal, which is 12-15 meters (39-49 feet) long, and that it will only really be at home again if it reaches the Atlantic Ocean.

The whale was spotted stuck on an underwater sandbank in Timmendorfer Strand, a popular resort town, on Monday morning. Initial efforts to free it and coax it back toward deeper water, including using coast guard and fire department boats to create large waves, were unsuccessful. The Baltic Sea lacks strong tides that could have freed the whale.

On Thursday morning, rescuers brought in excavators to dig a trench in front of the whale’s head, while Lehmann approached the animal and guided the digging. They worked until well after sunset, but hadn’t quite been able to get the whale out by the time they had to stop work for the night.

Early on Friday morning, the whale was on its way out of Lübeck Bay, where Timmendorfer Strand is located, said Stephanie Gross of the Institute of Terrestrial and Aquatic Wildlife Research. She said the massive mammal, which was about 300 meters (nearly 1,000 feet) off the coast, was being accompanied by a coast guard ship and several boats.

Experts assume that the whale is a young male, as males, unlike females, tend to migrate. It also appeared to be the same whale that was spotted several times in the port of Wismar, east of Timmendorfer Strand, in recent weeks.

It was not immediately clear why the whale was in the area and got stranded.

Humpback whales aren’t native to the Baltic. This one faces a journey of several hundred kilometers (miles) through German and Danish waters if it is to reach the North Sea.