The theater/temple of San Nicola near Caserta, Italy, was rediscovered by chance in 2000 by a paraglider. It's 2100 years old and hosts a theater and a temple on the same hill 520 high
Nah, they are all over. There's enough of them that the ancient aliens people always end up talking about them. Famous one in Eastern Europe. The difference is that those have strata layers, not blocks of stone
Mountains that look like pyramids are pretty common. Normally they aren't quite that straight, but it does happen. AFAIK we scan them these days to see if anything is there before bothering to dig
This site, "the ancient theater archive" says the hangglider guy was used to seeing glimpses of ruins covered by foliage, but there had been a recent brush fire that made it more obvious.