  We report the first detection of a methanol maser in the 10(1)-9(2)A-
transition at 23.4 GHz, discovered during the H2O southern Galactic Plane
Survey (HOPS) with the 22-m Mopra radio telescope. In the region covered by
HOPS, the 23.4 GHz maser was found at only one location, G357.97-0.16, which
was also a prominent source of maser emission in the J(2)-J(1)E series near 25
GHz. The Australia Telescope Compact Array (ATCA) was used to follow up these
detections at high angular resolution and prove the maser nature of the
observed emission. The analysis shows that the new methanol maser at 23.4 GHz
is a class I maser, which has properties similar to the 9.9 and 25 GHz masers
(i.e. traces strong shocks with higher than average temperature and density).
All class I masers were found to originate at the same spatial location (within
the measurement uncertainty of 0.5 arcseconds) in the vicinity of the dominant
infrared source, but at a clearly distinct position from nearby OH, H2O and
class II methanol masers at 6.7 GHz. All maser species are distributed
approximately on a line, but it is not clear at present whether this has any
physical significance. We also detected a weak (1.3 mJy) continuum source at 25
GHz near the OH maser (at the most northern site, associated with a class II
methanol maser and an H2O maser renowned for its extremely wide spread of
velocity components). The continuum source has not been reported at lower
frequencies and is therefore a candidate hypercompact HII region. We also used
the ATCA to find the strongest and only fifth known 9.9 GHz maser towards
G357.97-0.16 and another 23.4 GHz maser towards G343.12-0.06 not seen in HOPS.
