“We penetrated deeper and deeper into the heart of darkness”

"To Shakespeare"
Poems (1851)
Context: The soul of man is larger than the sky,
Deeper than ocean, or the abysmal dark
Of the unfathomed center. Like that ark,
Which in its sacred hold uplifted high,
O'er the drowned hills, the human family,
And stock reserved of every living kind,
So, in the compass of the single mind,
The seeds and pregnant forms in essence lie,
That make all worlds. Great poet, 'twas thy art
To know thyself, and in thyself to be
Whate'er Love, Hate, Ambition, Destiny,
Or the firm, fatal purpose of the Heart
Can make of Man. Yet thou wert still the same,
Serene of thought, unhurt by thy own flame.

Source: Heartbreak House

Fragment No. 91
Blüthenstaub (1798)

Source: Light in the Dark/Luz en lo Oscuro: Rewriting Identity, Spirituality, Reality (2015), p. 111

Bastian, Jonathan (host). "Bittersweet: Susan Cain on the joy of sweet sorrow". KCRW.com (Los Angeles). May 21, 2022.

A Tree Telling of Orpheus (1968)
Context: He told of journeys,
of where sun and moon go while we stand in dark,
of an earth-journey he dreamed he would take some day
deeper than roots...
He told of the dreams of man, wars, passions, griefs,
and I, a tree, understood words – ah, it seemed
my thick bark would split like a sapling's that
grew too fast in the spring
when a late frost wounds it.

The Ancient And Modern Muses

Source: Fell

This passage contains some phrases King later used in "Where Do We Go From Here?" (1967) which has a section below.
1950s, Loving Your Enemies (Christmas 1957)
Variant: Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate, only love can do that.
Source: Mentioned in "Out of Osama's Death, a Fake Quotation Is Born" by Megan McArdle, The Atlantic (May 2011) http://www.theatlantic.com/national/archive/2011/05/out-of-osamas-death-a-fake-quotation-is-born/238220/, and widely distributed on twitter http://twitter.com/#!/jmadly/status/65314784136011776 as a quote of King, after the death of Osama bin Laden, the first sentence is one written by Jessica Dovey http://i.imgur.com/cqtjw.jpg on her Facebook page, which became improperly combined by others with genuine statements of King, whom she quoted, and which occur in Strength to Love (1963), Ch. 5 : Loving your enemies, and in Where Do We Go from Here : Chaos or Community? (1967), p. 62.
For the full story see "Anatomy of a Fake Quotation" by Megan McArdle, The Atlantic (May 3, 2011) http://www.theatlantic.com/national/archive/2011/05/anatomy-of-a-fake-quotation/238257/ and for the Facebook version of the quote see Did Martin Luther King, Jr. say that “I will not rejoice in the death of one, not even an enemy”? at skeptics.stackexchange.com http://skeptics.stackexchange.com.
Context: Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate; only love can do that. Hate multiplies hate, violence multiplies violence, and toughness multiplies toughness in a descending spiral of destruction.
Context: Let us move now from the practical how to the theoretical why: Why should we love our enemies? The first reason is fairly obvious. Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate; only love can do that. Hate multiplies hate, violence multiplies violence, and toughness multiplies toughness in a descending spiral of destruction. So when Jesus says "love your enemies," he is setting forth a profound and ultimately inescapable admonition. Have we not come to such an impasse in the modern world that we must love our enemies-or else? The chain reaction of evil-Hate begetting hate, wars producing more wars-must be broken, or we shall be plunged into the dark abyss of annihilation.
Context: I mourn the loss of thousands of precious lives, but I will not rejoice in the death of one, not even an enemy. Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness: only light can do that. Hate cannot drive out hate: only love can do that.