For a long time, Thailand's Similan Islands were ranked among the best diving destinations in the world, but tsunamis and stints of abnormally warm water have done lasting damage to the region's lush reefs. The local corals are now slowly recovering – although not without setbacks caused by thoughtless tourists.

While leading divers near famed Turtle Rock, the crew at Sign Scuba made a disturbing discovery. A large brain coral (Leptoria phrygia) had been etched with Korean writing. 

"No conscience," Sookkasame Jarupong, who spotted the vandalism, wrote on Facebook. "I was super bummed to find the name written."

While one act of vandalism might not seem particularly problematic, local officials are concerned that this incident could prompt other tourists to follow suit. Images of heart-covered trees come to mind here, but unlike arboreal art, this kind of damage affects corals' growth. 

Stony (or "reef-building") corals look like rocks, but they're actually living colonies of tiny animals called polyps. A single colony can have hundreds of thousands of individual members. They're each just millimetres tall, yet together, they can create sizeable structures over time. 

As new polyps settle into a coral colony, they secrete tough calcium carbonate on top of the skeletons left behind by previous generations. This means living polyps are found only on the outermost layer of the structure – precisely the part that's been scratched off here.

And unlike many other species, brain coral polyps share common walls – that's what gives the colony its unique, maze-like appearance. Judging by the photos, the diver managed to scrape both into the calcium carbonate and the living tissues. This suggests a dive knife was possibly used to do the etching.

"This is a very negative attitude and sets a bad example for other divers who frequent the island," a representative from Sign Scuba told the Phuket Gazette. "On behalf of the diving community, I request officials to keep a closer eye on such activities and enforce rules more vigilantly before this issue becomes worse."

Outcry on social media echoes that sentiment, but the Sign Scuba team emphasises that the bad behaviour of one diver doesn't implicate everyone. "Normally, Korean tourists follow all rules and regulations," they said. 

Around a third of the world's reef-building corals are threatened with extinction, and brain corals like this one are extremely slow growing. The oldest known colony is thought to be almost 2,000 years old, but it measures just 1.8 metres (6ft) tall. These stony structures provide critical shelter for hundreds of reef species, so if you see one, keep your distance and carve it into your memory instead.


Top header image: Philippe Bourjon, Wikimedia Commons