The heart grieves what the eye does not see. The nightmare of the Marmara Sea, mucilage, or "sea snot," reappeared off the coast of Maltepe, Istanbul on Tuesday, with experts and activists resuming warnings that the environmental catastrophe might affect the region again this summer with the current rate of pollution and warming.

The unsettling news came two days after scientists conducting underwater studies off Princes' Islands in the Marmara Sea found that last year's breakout greatly affected marine life, including corals and sponges.

The mucilage problem, which has plagued the surface of the Marmara, reached a level that could be described as catastrophic last year. The natural phenomenon, blamed on the increase in organic pollutants and rise in sea temperatures, greatly affected tourism and fishing.

Turkish authorities had announced the 22-point Marmara Sea Action Plan to tackle the surge. After an intense sea cleanup campaign, Turkish authorities stated that the sea had been cleared of mucilage.

No presence of mucilage was found in the underwater images taken in Maltepe, whereas aerial images captured mucilage layers on the sea surface.

In a short distance from the coast of Maltepe, researchers from the Istanbul University Faculty of Aquatic Sciences have been conducting an underwater study on the effects of mucilage, and their findings drew a grim picture.

Underlining the richness of the area in terms of sponges, especially of a point to the south of Büyükada, associate professor Nur Eda Topçu said: "It was a place where you can see every color on a color scale in a small area. Now everything looks brown. The number of sponges has also diminished greatly," she said.

Backed by the Scientific and Technological Research Council of Turkey (TÜBITAK), the research, which is being carried out at nine diving stations around the islands, is among the 37 projects supported by the body as part of its mucilage research special call. It aims to find out what survived after the mucilage phenomenon, how many are out there and where are they located.

Topçu told the Anadolu Agency (AA) that the region has seabed areas that could be described as a "coral garden" or a "forest in terms of thickness, and it is home to some species only found in the Mediterranean. Starting from 2015, a breaking point occurred for the creatures living under the seabed and mass deaths occurred in sponges and corals as seafront construction in the region increased, she explained.

"Especially red gorgonian (soft coral) is a species that has been decreasing in numbers since 1970s due to fishing. It became very scarce later due to other causes, especially seafront activities. Now, it is almost nonexistent due to mucilage," Topçu said, underlining that red gorgonians are doomed to be erased from the region since they are a long-lived, slow-growing species with low ability to multiply.

"Not everything is lost, there is still life. If we give a chance to Marmara, I think there could be a fast recovery," Topçu said, noting that if a large mucilage outbreak occurs again, it may mean the death of all remaining seabed creatures.

Associate professor Bülent Topaloğlu stated that they detected 20 species of sponge in the Marmara Sea in earlier studies, but their numbers likely dropped after the mucilage breakout.

"It is as if a massacre occurred on the seabed. That's what the underwater images show. It will be possible for us to come up with more concrete results after analyses," he said.

Mucilage clogs pores of creatures living through filtering nutrients in the water, meaning death for sponges, Topaloğlu explained.

Thick layers of mucilage were also seen at the Dardanelles a week earlier despite the strait's strong currents. Documenting the incident on his fishing boat, Evren Kızoğlu, chairperson of Çanakkale City Council and mukhtar of the city's Cevatpaşa neighborhood, had told Ihlas News Agency (IHA) that what was witnessed as blurriness underwater a couple of days ago had thickened and hit the surface.

"The Marmara Sea is entirely ours and and is surrounded by our country. We've been told that we are facing the danger of its destruction. Our ministry has taken steps and the Marmara Union of Municipalities has held meetings. It is clear that inspections need to be tightened, meaning advanced biological treatment facilities should be used actively and waters should not be disposed of through deep sea discharge even if they were treated," Kızoğlu said.

The delicate situation in the Marmara Sea also alarmed the public, with residents of the port town of Gemlik in the Bursa province launching a marine protection campaign spearheaded by the local municipality last week against plans to set up a deep sea discharge system by a chemicals factory.

After a press release on Feb. 14, Gemlik municipality urged locals to sign a petition to protect the eponymous gulf, which is dotted with olive orchards and resorts along with serving as an important industrial and commercial hub. Mayor Mehmet Uğur Sertaslan said that the region, once known for sea sponge farming, has come to a point where even the health effects of swimming there are being discussed.

"Mucilage, which occurred last year after the weather got warm, served as an indicator of how the Marmara Sea has been damaged by pollutants. Now this deep sea discharge system planned for our district seeks to draw in 5,500 cubic meters (1.45 million gallons) of water per hour – seven times higher than all Gemlik uses in a day – for cooling, and then release the hot water to the sea. Of course, we know that different amounts of chemicals including chlorine will end up in the sea," he said.

"This system will increase the temperature of the Gulf of Gemlik by 1 degree Celsius (1.8 degrees Fahrenheit) in the winter and 2 degrees (Celsius) in the summer. Temperatures in the Sea of Marmara have increased by 2.5 degrees Celsius in the last 30 years. The increase in seawater temperatures and the use of chemicals will affect the region from the Gulf of Gemlik to the Kapıdağ Peninsula in Balıkesir. The Marmara Sea no longer has a defense against new polluting effects," the mayor added, calling on citizens to support the petition.