  Our existence in the Universe resulted from a rare combination of
circumstances. The same must hold for any highly developed extraterrestrial
civilisation, and if they have ever existed in the Milky Way, they would likely
be scattered over large distances in space and time. However, all
technologically advanced species must be aware of the unique property of the
galactic centre: it hosts Sagittarius A* (Sgr A*), the closest supermassive
black hole to anyone in the Galaxy. A civilisation with sufficient technical
know-how may have placed material in orbit around Sgr A* for research, energy
extraction, and communication purposes. In either case, its orbital motion will
necessarily be a source of gravitational waves. We show that a Jupiter-mass
probe on the retrograde innermost stable circular orbit around Sgr A* emits,
depending on the black hole spin, at a frequency of $f_{GW} = 0.63 - 1.07$ mHz
and with a power of $P_{GW}=2.7 \times\, 10^{36} - 2.0 \times\, 10^{37}$ erg/s.
We discuss that the energy output of a single star is sufficient to stabilise
the location of an orbiting probe for a billion years against gravitational
wave induced orbital decay. Placing and sustaining a device near Sgr A* is
therefore astrophysically possible. Such a probe will emit an unambiguously
artificial continuous gravitational wave signal that is observable with
LISA-type detectors.
