Something like 15 million years ago. That's right, even before disco.
And the world was a much different place. Global temperatures were 5 to 10 degrees higher and sea levels were 75 to 20 feet higher.
How do we know? you ask.
It used to be we only knew about atmospheric levels going back about 800,000 years -- a blink of an eye in geographic time -- by studying bubbles of air trapped in long-frozen Antarctic ice.
Now comes the very clever Aradna Tripati, who has peered back further in time by looking at sea shells.
Specifically, she's charted the ratio of boron to calcium in the shells to figure out what the atmosphere was like when they were formed. That time travels a good 20 million years.