Hi. I certainly have heard that story. Jung tells it in his autobiography. I think rather than metaphysics, the conversations were more about precognition and parapsychology.

Regarding the Red Book, I think there is more than an “underlying belief if we go too far we won't come back”. One understanding of psychosis and schizophrenia is that the ego has become overwhelmed by the contents of the unconscious, so it is certainly possible to get lost in the inner world. One therefore needs an extraordinarily strong ego to make such a journey. That is perhaps why those who undertake such a journey are described as 'heroes' (Joseph Campbell). Myths often make reference to this. The strong chance that Theseus would not be able to find his way out of the labyrinth (of the psyche) without the help of Ariadne (his anima figure) is well known.

In a separate story Hercules has to visit the underworld in order to rescue Theseus who is trapped there.

In more recent times, in Goethe's play Faust, the main character has to make such a journey. Here's the relevant passage from an online translation Faust - Johann Wolfgang von Goethe (alchemywebsite.com) :

Mephistopheles. The heathen-folk do not concern me.

They occupy a hell that's all their own.

But help there is.

Faust. Quick! Tell its history!

Mephistopheles. Not glad do I reveal a loftier mystery-

Enthroned sublime in solitude are goddesses;

Around them is no place, a time still less;

To speak of them embarrasses.

They are the Mothers!

Faust [terrified]. Mothers!

Mephistopheles. Do you fear?

Faust. The Mothers! Mothers! Strange the word I hear.

Mephistopheles. Strange is it. Goddesses, to men unknown,

Whom we are loath to name or own.

Deep must you dig to reach their dwelling ever;

You are to blame that now we need their favour.

Faust. Whither the way?

Mephistopheles. No way! To the Unexplorable,

Never to be explored; to the Unimplorable,

Never to be implored. Are in the mood?

There are no locks, no bars are to be riven;

Through solitudes you will be whirled and driven.

Can you imagine wastes and solitude?