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The New York Post published an article in 2016 titled Obituary: Great Barrier Reef (25 Million BC-2016).

The Great Barrier Reef of Australia passed away in 2016 after a long illness. It was 25 million years old.

I have a hard time even understanding what this means.

Is this a single organism?

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No, the Great Barrier Reef is not dead.

According to Exploring the hidden shallows: extensive reef development and resilience within the turbid nearshore Great Barrier Reef (2016):

Mean coral cover on Australia’s Great Barrier Reef (GBR) has reportedly declined by over 15% during the last 30 years. Climate change events and outbreaks of coral disease have been major drivers of degradation, often exacerbating the stresses caused by localised human activities (e.g. elevated sediment and nutrient inputs).

See also The Great Barrier Reef is not actually dead

Also, the reef hasn't been alive for 25 million years. Instead, according to the Great Barrier Reef Marine Park Authority:

The current Great Barrier Reef structure started growing on top of the old reef platform about 9000 years ago when the sea levels rose at the end of the last Ice Age

So, basically, there was coral (but not a reef) as long ago as 25 million years, but there was no reef structure until 600,000 years ago, and it hasn't been continuously alive since then, but instead only for the last 9000 years.

See also the Australian government article How the Great Barrier Reef was formed - Twenty thousand years ago :

20,000 years ago, the world was starting to emerge from the most recent ice age. The sea level around the Australian coast was then about 120 meters lower than it is today.
...
Reefs are only formed in shallow water where there is adequate light for the organisms that build them. Reefs do not form at depths greater than about 30 meters.

So the reef was formed in shallow water that was dry land 20,000 years ago.

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