Arthur C Clarke placed three monoliths in our solar system (one on Earth) as a way to let the aliens know how we were progressing.
Possibly this was extraordinarily prescient. There appear to be monoliths on Mars, and also on Earth that could have extraterrestrial origin. They could be linked – we will need to see the Martian ones up close to be sure.
Gobleki Tepe has a few monoliths, two are 5.5m high. They were buried until their recent discovery. They were deliberately buried, as if to say it might take an advanced culture to find them. They are 10,000+ years old.
Gobekli Tepe Pillar
Mars and its moon also seem to have non-natural monoliths. As we know from the “face on Mars”, blurry photographs can suggest more than reality. We can only find out by taking a closer look.
Phobos (moon of Mars, that could be artificial…)
https://en.wikipedia.org/wiki/Phobos_monolith
Phobos Mars Moon Monolith
And on Mars itself
https://en.wikipedia.org/wiki/Mars_monolith
Monolith on Mars
Scientists don’t believe these were created by erosion, like similar monoliths on Earth.