Document · 1963
Sagan 1963: 'Direct contact among galactic civilizations by relativistic interstellar spaceflight'
Planetary and Space Science 11, pp. 485-498 (NASA NTRS 19630011050). On p. 497 Sagan says Earth may have been visited ~10^4 times, artifacts may survive, a base may be 'maintained (possibly automatically)', 'the Moon seems one reasonable alternative', and high-resolution reconnaissance 'particularly of the back side' should bear this in mind. Max quotes it accurately.
Connected
Carl Sagan personParacelsus C crater (Moon) placeDavies & Wagner: 'Searching for alien artifacts on the Moon' (2013) document