The environmental organisation dropped the rocks in an area of the South West Deeps Conservation Zone, some 190 kilometres off the English coast.
Activists dropped giant boulders into the English Channel off of Land's End in Cornwall in the latest attempt by Greenpeace UK to try and prevent destructive industrial fishing.
The environmental organisation dropped the giant rocks in an area of the South West Deeps Conservation Zone that they claim suffers from overfishing by large, bottom-trawling boats.
Anna Diski, Greenpeace's UK oceans campaigner, says that placing these boulders stops destructive trawlers from being able to "operate in the area by making it impossible for them to drag their heavy fishing gear along the seabed, destroying the habitat and disturbing the carbon."
The action comes after the latest round of UN talks to try to secure protection for marine life in international waters broke up without agreement.
'Small act is a proof that it can be done'
Adrián Araúz, the captain of the Arctic Sunrise, the ship used in the operation, thinks that if the government had the willingness to protect some areas, "they could do it, they have the power".
"So the small act that we are doing is proof that it can be done," Araúz.
Greenpeace said that the 4,600-square kilometre patch at the South West Deeps is one of the most heavily fished so-called Marine Protected Areas in the UK.
Figures from the Global Fishing Watch monitoring agency show that 110 vessels, more than half of them from France, fished for nearly 20,000 hours in the area in the 18 months to July.