  The first astronomical detection of the metastable ($J,K$) = (18,18) line of
NH3 is reported. With 3130 K above the ground state, this is the NH3 line with
by far the highest energy detected in interstellar space. It is observed in
absorption toward the galactic center star forming region Sgr B2. There is a
clear detection toward Sgr B2(M) and a likely one toward SgrB2(N). An upper
limit for emission is determined for Orion-KL. If we combine the (18,18) line
results from Sgr B2(M) with the previously measured (12,12) absorption line, we
find a rotation temperature of >1300 K for the absorbing cloud. This is at
least a factor of two higher than previously derived values from less highly
excited ammonia lines, giving a lower limit to the kinetic temperature. There
is a hot low density gas component in the envelope of SgrB2. It is possible
that the (18,18) line arises in this region. The radial velocity of the low
density, hot envelope is the same as that of the dense hot cores, so the
(18,18) line could also arise in the dense hot cores where non-metastable (J>K)
absorption lines from energy levels of up to 1350 K above the ground state have
been observed. A discussion of scenarios is presented.
