You may well have heard that Australia’s iconic Great Barrier Reef is dying as warmer and more acidic waters bleach the system’s vibrant coral reefs. In fact, a heatwave killed nearly a third of the system’s corals in 2016.

Now, scientists writing in the journal Nature Geoscience have discovered the reef has bounced back from near-extinction five times in the last 30,000 years. The current stresses, however, are probably far more intense than those felt in the past.

"I have grave concerns about the ability of the reef in its current form to survive the pace of change caused by the many current stresses and those projected into the near future," study author Jody Webster from the University of Sydney said in a statement.

The international team looked beyond the boundaries of today’s reef to find places where ancient coral may have grown. Then, they drilled down into the sea floor, collecting 30,000 years’ worth of fossilized coral and sediments. These rock cores revealed the coral’s dramatic past.

As sea levels rose and fell, coral reef migrated. Low sea levels 30,000 and 22,000 years ago killed coral by air exposure. The remaining reef shifted seawards and eventually bounced back.

Rising sea levels—like those we see today—killed off the coral twice between 13,000 and 17,000 years ago. This time, coral inched close to land to survive. The reef system, the scientists think, migrated up to 60 inches a year in the face of a changing environment. The last of the five great die-offs ocurred about 10,000 years ago, and was likely caused by a huge influx of sediment, a reduction in water quality, and a general sea level rise. The modern reef emerged some 9,000 years ago.

Although these historical challenges are similar to those facing the system today, “don’t expect reefs to be able to bounce back quickly,” Mark Eakin, a coral reef ecologist at the National Oceanic and Atmospheric Administration (NOAA) who was not involved in the study, told Science.

The rate of sea surface temperature and water acidification change was probably slower in the past, Webster told the AFP. The reef, he said, is now probably battling changes faster than it has seen before.

Coral reefs—living nurseries for other marine life—become far more vulnerable to death and disease when bleached. Australia’s Great Barrier Reef, which is a UNESCO World Heritage Site, is home to more than 1,500 species of fish, as well as sharks, marine turtles, rays and marine mammals like the dugong, according to the World Wildlife Fund.

The reef system may be due for another die-off some time in the next few thousand years, “if it follows its past geological pattern,” Webster told AFP. “But whether human-induced climate change will hasten that death remains to be seen."