Huge Elongated skulls, aliens. Out of place artifacts, aliens. TPTB, aliens. Ritual mutilation/sacrifice, aliens. Mysterious human origin, aliens. Complex ancient structures, aliens. Religious/spiritual entities, aliens. Atmospheric phenomena, aliens
the list goes on, feel free to add.
EXTRA (beyond accessibility) TERRA (earth). Anything others prevent one from accessing represents extra terrestrial.
A'LIEN, adjective [Latin alius, another.] For one (perceiving) within all (perceivable) everything else represents another one...aliens.
SPACE, noun [Latin spatium, spatior, to wander. ] Being temporary form (life) within ongoing flow (inception towards death) allows one to wander within all.