  When the cosmic star formation history peaks (z ~ 2), galaxies vigorously fed
by cosmic reservoirs are gas dominated and contain massive star-forming clumps,
thought to form by violent gravitational instabilities in highly turbulent
gas-rich disks. However, a clump formation event has not been witnessed yet,
and it is debated whether clumps survive energetic feedback from young stars,
thus migrating inwards to form galaxy bulges. Here we report spatially resolved
spectroscopy of a bright off-nuclear emission line region in a galaxy at z =
1.987. Although this region dominates the star formation in the galaxy disk,
its stellar continuum remains undetected in deep imaging, revealing an
extremely young (age < 10 Myr) massive clump, forming through the gravitational
collapse of > 10$^9$ M$_{\odot}$ of gas. Gas consumption in this young clump is
> 10 times faster than in the host galaxy, displaying high star formation
efficiency during this phase, in agreement with our hydrodynamic simulations.
The frequency of older clumps with similar masses coupled with our initial
estimate of their formation rate (~ 2.5 Gyr$^{-1}$) supports long lifetimes (~
500 Myr), favouring scenarios where clumps survive feedback and grow the bulges
of present-day galaxies.
