One hundred thousand wizards gasped as the two Seekers, Krum and Lynch, plummeted through the center of the Chasers, so fast that it looked as though they had just jumped from airplanes without parachutes.
Pigwidgeon plummeted twelve feet before managing to pull himself back up again; the letter attached to his leg was much longer and heavier than usual — Harry hadn’t been able to resist giving Sirius a blow-by-blow account of exactly how he had swerved, circled, and dodged the Horntail.
Harry soared higher in a circle; the Horntail was still following his progress; its head revolving on its long neck — if he kept this up, it would be nicely dizzy — but better not push it too long, or it would be breathing fire again Harry plummeted just as the Horntail opened its mouth, but this time he was less lucky — he missed the flames, but the tail came whipping up to meet him instead, and as he swerved to the left, one of the long spikes grazed his shoulder, ripping his robes —…
There are no more uses of "plummet" in the book.
Show samples from other sources
The stock market plummeted
The car rolled down the hill and plummeted off the cliff.