  Today's most precise time and frequency measurements are performed with
optical atomic clocks. However, it has been proposed that they could
potentially be outperformed by a nuclear clock, which employs a nuclear
transition instead of the atomic shell transitions used so far. By today there
is only one nuclear state known which could serve for a nuclear clock using
currently available technology, which is the isomeric first excited state in
$^{229}$Th. Here we report the direct detection of this nuclear state, which is
a further confirmation of the isomer's existence and lays the foundation for
precise studies of the isomer's decay parameters. Based on this direct
detection the isomeric energy is constrained to lie between 6.3 and 18.3 eV,
and the half-life is found to be longer than 60 s for
$^{229\mathrm{m}}$Th$^{2+}$. More precise determinations appear in reach and
will pave the way for the development of a nuclear frequency standard.
