The riddle
Before Mount Everest Was Discovered, What Was the Tallest Mountain?
Measuring does not move a mountain
Everest was the tallest mountain before anyone measured it, and it is the tallest now. Surveyors found out which peak was highest, but finding out did not change a single foot of rock. The riddle gets listeners to picture a different champion that "used to" hold the title, when the real change was only in what people knew. People lived in the region long before any survey, so "discovered" here only means that surveyors identified the peak as the highest.
What was believed before
Until 1852, Kangchenjunga was assumed to be the highest mountain on Earth. Work by the Great Trigonometrical Survey of India in 1849 showed that the peak then called Peak XV was higher, and in 1856 it was officially announced that Kangchenjunga ranks third. The belief changed. The mountains stayed where they were.
Where the name comes from
Andrew Waugh, who followed Sir George Everest as Surveyor General of India, proposed naming Peak XV after his predecessor. Everest himself objected to the idea. The Royal Geographical Society adopted the name Mount Everest in 1865.
Tallest or highest
Everest is the highest mountain above sea level, measured in 2020 by a joint survey of Nepal and China at 8,848.86 meters, or 29,031.7 feet. Measured from its base on the ocean floor, however, USGS calls Mauna Kea in Hawaii the tallest, with a summit about 4,207 meters above the sea. The riddle means above sea level, as most people do.