At least four versions of the map are known to exist:
Version 0.0 (ACHERON.WAD), released June 10, 1994.
Version 1.666 (ACHERON6.WAD), released September 23, 1994.
Version 2.0 (ACHERON2.WAD), released October 12, 1994.
Version 2.2 (ACHRON22.WAD), released July 5, 1995.
The first two WADs replace E1M3; the other two replace MAP03. The level also appeared in Maximum Doom as ACHERON2.WAD, as MAP15 of R.WAD, as MAP15 of RALPHII.WAD, as E2M7 of RALPHNEW.WAD, and as MAP25 of WADPAK4.WAD.
If you want to get the yellow key, you have to go around the slime pit and press 'use' on the far side of the pit until the floor with the key raises, otherwise if you go into the pit to get it, you will be stuck there (however, the sector that the key is on doesn't hurt you).
An alternate release of version 2.0, labeled 2.0a, occurred on December 5, 1994. The two WADs are identical apart from the ENDOOM lump.
Inspiration and development
Crossing Acheron is the sequel to Dante's Gate. Although released singly, both were originally conceived as part of a full episode replacement. E1M3 was chosen over E1M2 as the map slot because E1M3's stock music better fit the overall theme.
At the time, Dr. Sleep said that the original release of Dante's Gate was too easy, and he wanted Crossing Acheron to have a faster pace (in addition to his customary emphasis on texture placement and atmosphere). Subsequent revisions added new areas and monsters, increasing the difficulty further.
Acheron is a river in hell, according to Dante's story (and the Greek spiritual traditions preceding it. In Greek mythology, Acheron was the first of the five rivers of the underworld that the dead had to cross.). Dr. Sleep tried to create a river as an architectural feature in this map, but couldn't come up with one which looked sufficiently realistic.