  $^{180m}$Ta is the longest-lived metastable state presently known. Its decay
has not been observed yet. In this work, we report a new result on the decay of
\mTa obtained with a $2015.12$-g tantalum sample measured for $527.7$ d with an
ultra-low background HPGe detector in the STELLA laboratory of the Laboratori
Nazionali del Gran Sasso (LNGS), in Italy. Before the measurement, the sample
has been stored deep-underground for ten years, resulting in subdominant
background contributions from cosmogenically activated $^{182}$Ta. We observe
no signal in the regions of interest and set half-life limits on the process
for the two channels EC and $\beta^-$: $T_{1/2,~\mathrm{EC}} > 1.6 \times
10^{18}$ yr and $T_{1/2,~\beta^-} > 1.1\times 10^{18}$ yr ($90$\% C.\,I.),
respectively. We also set the limit on the $\gamma$ de-excitation / IC channel:
$T_{1/2,~\mathrm{IC}} > 4.1 \times 10^{15}$ yr ($90$\% C.\,I.). These are, as
of now, the most stringent bounds on the decay of $^{180m}$Ta worldwide.
Finally, we test the hypothetical scenarios of de-excitation of $^{180m}$Ta by
cosmological Dark Matter and constrain new parameter space for
strongly-interacting dark-matter particle with mass up to $10^5$ GeV.
