### Confusing. "Muons are

Confusing.
"Muons are particles created high up above the Earth's surface by cosmic rays hitting the atmosphere. They can be detected in large numbers at the Earth's surface. They travel at around 0.998 times the speed of light, but they don't live for very long - typically, they decay after only $2 \times 10^{-6} = 0.000002$ seconds. "

"In our frame of reference the lifetime of the muon is extended from $2 \times 10^{-6}$ seconds to $2 \times 10^{-6}/\sqrt {1 - \frac{(0.998 \times c)^2}{c^2}}= 31.7 \times 10^{-6}$ seconds."

Where did 0.000002 seconds come from/calculated or was measured in which frame.?