People have inhabited Europe for thousands of years, building new empires atop old ones. 

That’s precisely why the continent is such a fascinating place for underground exploration—travelers can look into the past through well-preserved subterranean cities, hideouts, and tombs. Discover subterranean secrets, from Rome’s bone-covered catacombs to Istanbul’s sunken palace.

 Read the full article here: https://www.nationalgeographic.com/travel/article/underground-sites