A study of the metastable (J,K)=(18,18) line of ammonia (NH3) is presented aiming at better defining the physical properties of molecular gas in extreme environments. The spectra were collected with the Effelsberg 100-m telescope and are analyzed in combination with other data using a rotation diagram (Boltzmann plot). The first astronomical detection of the metastable (J,K) = (18,18) line of NH3 is reported. This is the NH3 line with by far the highest energy, 3130 K above the ground state, 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 Sgr B2 (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, T_rot, 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. T_rot gives a lower limit to the kinetic temperature. There is a hot low density gas component in the envelope of Sgr B2. 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.