In SRB2 version 1.09.4, Red Volcano Zone Act 1 was hardcoded so that the game would crash if you typed "map map16" in the console. It was also stored in the "soar.dta" file.
If you want to access it normally:
You have to beat the game three times (which is made easier by making a save file the moment you reach Castle Eggman Zone Act 3, then loading the file twice to beat it without having to play through Greenflower and Techno Hill again). Once that's done, you go to RedXVI's house in GFZ1 and jump on his house three times. He will then send you to "Hell" (RVZ1).