Scientists have identified safe areas for ships to drop anchor off Rathlin Island, in an attempt to protect the the area's unique habitat.
Cruise liners are calling at the island off the County Antrim coast because of its reputation as a bird sanctuary.
But in the last three years, divers have discovered that the bay is home to a unique form of "concrete seaweed".
Maerl is a coralline alga which takes calcium carbonate out of the seawater and creates a hard skeleton.
It is incredibly slow growing - the rate is around 1mm a year - and it interlocks to form an important nursery habitat for juvenile scallops and other fish.
Joe Breen, senior scientific officer with the Department of the Environment's Marine Division, leads the team which discovered the maerl and is working to protect it.
"We'd been looking for maerl for 30 years and had probably driven the boat over it 300 times," he said.
"We found it just by chance on a dive close to the harbour entrance. When we dropped down we couldn't believe it.
"When we identified the site, we did a very good survey so that we weren't going to be allowing the cruise ships to come in and put an anchor on top of something so unique."
The team identified two safe anchorages, each with a 250m radius away from the maerl.
When the cruise ships come in to Rathlin they speak to the harbour master at Ballycastle, who gives them a precise location to drop anchor.
Tourists are then transferred to the island by boat.
Liz Pothanikat is a scientific officer with the DoE. She said the presence of maerl is an indicator that the water quality off Rathlin is good.
"It is an algae, it needs light to photosynthesise so the water that we get around Rathlin is perfect for it," she added.
"It also needs quite strong currents and you'll find that it will create an almost 3-D interlocking structure that normal seaweeds and algae wouldn't grow in.
"This makes it an important nursery habitat for fish - there are lots of little holes for them to hide in."
Maerl has another fascinating side story.
Due to the way it creates a hard skeletal covering, it is currently being used in research related to bone reconstruction.
BBC Newsline will be broadcasting from Rathlin Island on Wednesday and Thursday at 18:30 BST on BBC One Northern Ireland.