From his Letters:
No. 135 -- I am at last after three weeks incessant labour of the most
exacting and dreariest sort (college related work just finished)...and also (if I can)
finding somewhere else to live and moving! This charming house has
become uninhabitable --- unsleepable-in, unworkable-in,rocked, racked
with noise, and drenched with fumes. Such is modern life. Mordor in
our midst. And I regret to note that the billowing cloud recently
pictured did not mark the fall of Barad Dûr, but was produced by its
allies -- or at least by persons who have decided to use the Ring for
their own (of course most excellent) purposes.
From the Unfinished Tales:
History of Galadriel & Celeborn -- But there was in Thranduil's heart
a still deeper shadow. He had seen the horror of Mordor and could not
forget it. If ever he looked south its memory dimmed the light of the
Sun, and though he knew that it was now broken and deserted and under
the vigilance of the Kings of Men, fear spoke in his heart that it was
not conquered for ever: it would rise again.
Cirion & Eorl -- So it came to pass that the head of the army of
Gondor had only drawn level with the Gates of Mordor (the Morannon)
when a great dust borne on a wind from the East announced the oncoming
of the enemy vangaurd. ... Ondoher was utterly unprepared to meet a
charge of horsemen and chariots in great weight. With his Guard and
his banner he had hastily taken up a position on a low knoll, but this
was of no avail.
This is undoubtedly the same low knoll that Aragorn would stand on much later.
The Wainriders came on in little order, still exultant and singing
songs of victory, seeing as yet no signs of any defenders to oppose
them, until they found that the road into Gondor turned south into a
narrow land of trees under the shadow of the dark Ephel Dúath, where
an army could march, or ride, in good order only down a great highway.
Before them it ran on through a deep cutting...
From Silmarillion:
Of the Rings of Power -- He came in secret, as has been told, to his
ancient kingdom of Mordor beyond the Ephel Dúath, the Mountains of
Shadow, and that country marched with Gondor upon the east. There
above the valley of Gorgoroth was built his fortress vast and strong,
Barad Dûr, the Dark Tower; and there was a fiery mountain in that land
that the Elves named Orodruin. Indeed for that reason Suron had set
there his dwelling long before, for he used the fire that welled there
from the heart of the Earth in his sorceries and in his forging; and
in the midst of the Land of Mordor he had fashioned the Ruling Ring.
Now Sauron prepared war against the Eldar and the Men of Westernesse,
and the fires of the Mountain were wakened again. Wherefore seeing the
smoke of Orodruin from afar, and perceiving that Sauron had returned,
the Númenoreans named that mountain anew Amon Amarth, which is Mount
Doom.
The Númenoreans indeed set a guard upon the land of Mordor, but none
dared dwell there because of the terror of the memory of Sauron, and
because of the Mountain of Fire that stood nigh to Barad Dúr; and the
valley of Gorgoroth was filled with ash.
And of course, Mordor was not all dreary and ashy.
Book VI, The Land of Shadow -- Neither [Sam] nor Frodo knew anything
of the great slave-worked fields away south in this wide realm, beyond
the fumes of the Mountain by the dark sad waters of Lake Núrnen; nor
of the great roads that ran away east and south to tributary lands...
...and the slaves of Mordor he released and gave to them all the lands
about Lake Núrnen to be their own.
From the Map:
We can see that Mordor is divided into two great regions, hemmed in on three sides by high mountains.
- In the northwestern corner is the Plateau of Gorgoroth. This is the ashy & noisome, working class province where all the Orc armies live
and train.
- In the south and east lies the broad breadbasket of Mordor, the great Plains of
Núrnen. We can see that Lake Núrnen lies at the bottom
of an endorheic basin. These are terminal or sink lakes where the
water does not flow to the ocean, but rather the volume of water that
flows in from its rivers balances against evaporation and seepage. We
might also suspect that agriculture of the type practiced in Mordor,
all machines and wheels and reeking mechanisms of various kinds, takes
up quite a lot of water from the few rivers that traverse the Plain.
Of the rivers of Núrnen, we know there are four and they are unnamed
and they flow, two from the north and two from the south, and empty
into the four quarters of the Lake. If it weren't for the nature of
the labour scheme in Sauron's empire, this would probably be a nice
quiet and lovely land in which to dwell.
Map kept at the Bodlein: