Finally brief mention of a fairly recent archaeological discovery: Pluto's Gate to Hell (the god of the underworld was named Pluto by the Romans and Hades by the Greeks).  Pluto's Gate was considered in classical times to be the entrance to the underworld (one of many perhaps) and was discovered in 2013 by Italian archaeologists at the ancient city of Hierapolis in southwestern Turkey. 

The site as it appears now (source of this photograph)	The site as it might have appeared in ancient times.  This photograph, credited to Francesco D'Andria, the Italian archaeologist that announced the discovery in March, 2013, is found in a news report from the National Geographic Society.

The "gate" was built on top of a cavern and, in ancient times, a mist of deadly vapors could be seen coming from the cave.  Here's a quote from the Slate article where I first read about the discovery: "Two millennia ago, visitors to Pluto's Gate could buy small birds or other animals (the sale of which supported the temple) and test out the toxic air that blew out of the mysterious cavern.  Only the priests, high and hallucinating on the fumes, could stand on the steps by the opening to hell.  They would sometimes lead sacrificial bulls inside, later pulling out their dead bodies in front of an awed crowd.

As the Greek geographer, philosopher, and prolific traveler Strabo, who lived from 64/63 B.C. to 24 A.D., so enticingly described it: 'This space is full of a vapor so misty and dense that one can scarcely see the ground.  Any animal that passes inside meets instant death.  I threw in sparrows and they immediately breathed their last and fell.' Can you guess what such a deadly gas might be escaping from Pluto's Gate?  The answer is(was) carbon dioxide.