Dropping the large granite stones into the Adlergrund, a protected marine area in the Baltic Sea, is an act of “pollution,” a spokesperson for the Federal Maritime and Hydrographic Agency said earlier in the day.

The environmentalist organization is said to be in violation of a law that bans the deposit of refuse and other materials in the high seas.

Despite being threatened with a penalty, Greenpeace continued with the stunt on Tuesday.

The organization said that fishermen and local authorities had been informed of the coordinates of the rocks, which activists have been dropping into the sea from the Beluga II vessel since Sunday.

The activists say that they have dropped roughly 60 boulders into waters off Ruegen, Germany’s largest island, around 14 sea miles from the shore.

The rocks are intended to obstruct fishermen from trawling the seabed, even though such fishing is legal.

The Adlergrund protected area measures around 234 square kilometres and consists of a large sandy plateau and reefs.

“What keeps us going is our understanding that we are giving nature its right to integrity,” said Greenpeace marine biologist Thilo Maack, who said the area’s conservation status only existed on paper.

Authorities also sought to ban a similar stunt off the German island of Sylt in 2008, but Greenpeace said it successfully challenged the decision in court.