  The majority of hydrogen in the interstellar medium (ISM) is in atomic form.
The transition from atoms to molecules and, in particular, the formation of the
H$_2$ molecule, is a key step in cosmic structure formation en route to stars.
Quantifying H$_2$ formation in space is difficult, due to the confusion in the
emission of atomic hydrogen (HI) and the lack of a H$_2$ signal from the cold
ISM. Here we present the discovery of a rare, isolated dark cloud currently
undergoing H$_2$ formation, as evidenced by a prominent "ring" of HI
self-absorption. Through a combined analysis of HI narrow self-absorption, CO
emission, dust emission, and extinction, we directly measured, for the first
time, the [HI]/[H$_2$] abundance varying from 2% to 0.2%, within one region.
These measured HI abundances are orders of magnitude higher than usually
assumed initial conditions for protoplanetary disk models. None of the fast
cloud formation model could produce such low atomic hydrogen abundance. We
derived a cloud formation timescale of 6$\times$10$^6$ years, consistent with
the global Galactic star formation rate, and favoring the classical star
formation picture over fast star formation models. Our measurements also help
constrain the H$_2$ formation rate, under various ISM conditions.
