It's a bonus cavern after you defeat Zeromus, iirc, I think it's available on the 3d PC version, Gameboy Advance Version, and the PSP version, it was originally gonna be started with the SNES but wasn't able to be added due to them running out of memory on their computers, but if I could I could try finding a rom editor and add it to the SNES version using the tilesets for the lunar caverns. The lunar ruins is practically like a continuation of the game.

These are GBA/PSP exclusive bonus content. --- FF4 3D Remakes does have few new cutscenes , 2 new bosses (strong ones) , but no new Dungeons. And we can't change party members. Mobile and PC Versions of FF4 3D Remakes has Hard mode , unlike the DS version. --- Final Fantasy 1,2,3,4,5 and 6 Pixel Remasters don't have the Bonus content, But are rebalanced versions of the NES (1,2 and 3) and SuperNES (4,5 and 6) releases. And has slightly different art style and has remixed musics and having the original musics. Switch and PS4 versions of FF Pixel Remasters does has options to gain 0 (zero) , Half, Double, Four times the EXP , Gil, AP (FF5 and 6) , Stat gains (FF2), Weapon and Spell EXP (FF2), As well as turning Enemy Encounters off or on at anytime). For those who are wondering or Don't remember.

@PhantomBlackStar thank you for that incredible extra information breakdown. That is one cool thing about trying different version. You may find something new or exciting. I am playing FF7 on Switch. the extra things make it a whole new experience and seeing what things have Changed or smoothed out.

@@TruMaverick , your welcome.

If I remember correctly, you have to defeat Zeromus with every character (Palom/Porom count as the same). Defeat it once with Cecil, Rosa, Kain, Rydia, and Edge, then save clear data, load it and defeat Zeromus again with Cecil, Yang, Cid, Edward, and Porom and save new clear data.