Man vs. Nature = Man vs. Himself
"The earth seemed unearthly.  We are accustomed to look upon the shackled form of a conquered monster, but there- there you could look at a thing monstrous and free.  It was unearthly, and the men were- No, they were not inhuman.  Well, you know, that was the worst of it- this suspicion of their not being inhuman.  It would come slowly to one.  They howled and leaped, and spun, and made horrid faces: but what thrilled you was just the thought of their humanity- like yours- the thought of your remote kinship with this wild and passionate uproar.  Ugly.  Yes, it was ugly enough; but if you were man enough you would admit to yourself that there was in you just the faintest trace of a response to the terrible frankness of that noise, a dim suspicion of there being a meaning in it which you- you so remote from the night of first ages- could possibly comprehend," (Conrad 51)
As I read through Heart of Darkness, I noticed the book was filled with imagery of nature overcoming man made technology.  Such as...
"...the muzzles of the long six-inch guns stuck out all over the low hull- the greasy, slimy swell swung her up lazily and let her down, swaying her thin masts.  In the empty immensity of earth, sky, and water, there she was, incomprehensible, firing into a continent...A heavy detonation shook the ground, a puff of smoke came out of the cliff, and that was all.  No change appeared on the face of the rock," (Conrad 27-28) 

The use of the phrase "firing into a continent" is especially powerful in that it shows the futility in this ridiculous action. 
"I came upon a boiler wallowing in the grass, then found a path leading up the hill.  It turned aside for the boulders, and also for an undersized railway truck lying there on its back with its wheels in the air.  One was off.  The thing looked as dead as the carcass of some animal.  I came upon more pieces of decaying machinery..." (Conrad 29).

"I nearly fell into a very narrow ravine...I discovered a lot of imported drainage-pipes for the settlement had been tumbled in there.  There wasn't one that was not broken.  It was a wanton smash-up," (Conrad 31).

These images were not thrown into the book just to fill up space.  They represent our new "civilized" nature, and what happens to it when it's thrown back into natural circumstances.  What would happen if, say, a comet were headed for the earth, and we knew that there would be no sunlight for three or four months?  How would our "civilized" society react to such a life or death situation?  The chances are, chaos would once again reign free like the vines crawling up some decripit car in a field.
I think that Conrad believed that in all our supposed progress, we have really changed little over the years.  The dark nature of man has not disappeared with the arrival of the grocery store, the butcher shop, the cheeseburger, or the use of militaristic force.  It simply has a new form and new mediums in which to be carried out.  Instead of being able to club one person over the head with a rock tied to a stick, we can now destroy millions of people at once with a fifty megaton nuclear device.  Instead of putting peoples heads on sticks we go to war and show it on CNN so everyone else can see what happens when you cross us  This progress is an illusion...
CNN
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