  Ellerman bombs are regions with enhanced Balmer line wing emission and mark
magnetic reconnection in the deep solar atmosphere in active regions and quiet
Sun. They are often found in regions where opposite magnetic polarities are in
close proximity. Recent high resolution observations suggest that Ellerman
bombs are more prevalent than thought before. We aim to determine the
occurrence of Ellerman bombs in the penumbra of sunspots. We analyze high
spatial resolution observations of sunspots in the Balmer H-alpha and H-beta
lines as well as auxiliary continuum channels obtained with the Swedish 1-m
Solar Telescope and apply the k-means clustering technique to systematically
detect and characterize Ellerman Bombs. Features with all the defining
characteristics of Ellerman bombs are found in large numbers over the entire
penumbra. The true prevalence of these events is only fully appreciated in the
H-beta line due to highest spatial resolution and lower chromospheric opacity.
We find that the penumbra hosts some of the highest Ellerman bomb densities,
only surpassed by the moat in the immediate surroundings of the sunspot. Some
penumbral Ellerman bombs show flame morphology and rapid dynamical evolution.
Many penumbral Ellerman bombs are fast moving with typical speed of 3.7 km/s
and sometimes more than 10 km/s. Many penumbral Ellerman bombs migrate from the
inner to the outer penumbra over hundreds of km and some continue moving beyond
the outer penumbral boundary into the moat. Many penumbral Ellerman bombs are
found in the vicinity of regions with opposite magnetic polarity. We conclude
that reconnection is a near continuous process in the low atmosphere of the
penumbra of sunspots as manifest in the form of penumbral Ellerman bombs. These
are so prevalent that they may be a major sink of sunspot magnetic energy.
