  Ultra Steep Spectrum (USS) radio sources have been successfully used to
select powerful radio sources at high redshifts (z>~2). Typically restricted to
large-sky surveys and relatively bright radio flux densities, it has gradually
become possible to extend the USS search to sub-mJy levels, thanks to the
recent appearance of sensitive low-frequency radio facilities. Here a first
detailed analysis of the nature of the faintest USS sources is presented. By
using Giant Metrewave Radio Telescope and Very Large Array radio observations
of the Lockman Hole at 610 MHz and 1.4 GHz, a sample of 58 USS sources, with
610 MHz integrated fluxes above 100 microJy, is assembled. Deep infrared data
at 3.6 and 4.5 micron from the Spitzer Extragalactic Representative Volume
Survey (SERVS) is used to reliably identify counterparts for 48 (83%) of these
sources, showing an average total magnitude of [3.6](AB)=19.8 mag.
Spectroscopic redshifts for 14 USS sources, together with photometric redshift
estimates, improved by the use of the deep SERVS data, for a further 19
objects, show redshifts ranging from z=0.1 to z=2.8, peaking at z~0.6 and
tailing off at high redshifts. The remaining 25 USS sources, with no redshift
estimate, include the faintest [3.6] magnitudes, with 10 sources undetected at
3.6 and 4.5 micron (typically [3.6]>22-23 mag, from local measurements), which
suggests the likely existence of higher redshifts among the sub-mJy USS
population. The comparison with the Square Kilometre Array Design Studies
Simulated Skies models indicate that Fanaroff-Riley type I radio sources and
radio-quiet Active Galactic Nuclei may constitute the bulk of the faintest USS
population, and raises the possibility that the high efficiency of the USS
technique for the selection of high redshift sources remains even at the
sub-mJy level.
