The vague, surreal cave of the past is a featureless terrain that seems to float in nothingness. The Phase Distorter III will heal you up for free, but you'll soon be too far away from it for it to be a practical source of healing.
(The music in this area contains a sample of the song "Deirdre" by the Beach Boys.)
Don't save your game with the Phase Distorter unless you want to be stuck here. You won't be able to explore things later if you save with the Phase Distorter.
Don't use PSI on the enemies here unless you have no choice. You need to conserve PSI for the final battle. There are a couple of Magic Butterfly spawn points in the cave, so look for those. Also, don't miss the Legendary Bat.
Bionic Krakens have a 1/128 chance of dropping the Gutsy Bat, so try for that if you wish.
Finally, you will eventually reach a narrow cave opening. Inside, you will find a grotesque, pulsating machine. Heal yourselves up to the maximum now. Also, give someone the Franklin Badge. Make your way up the long tubes, and you will reach an orb-shaped device. It will dilate, showing Ness's face. Then, Pokey will descend from above in a spider-like vehicle and will say that Giygas has become the embodiment of evil. And then, you must fight Giygas.