Explain to me the largest desert on Earth!#@$R#@%$

Well, probably you can find out more about this online. I read many articles previously and all what I remember for now is that the Sahara desert and the Arabian deserts were up to the late Holocene period wetter (more wet) as evident by the collected bones and cave art and rock art of both of the Sahara and the Arabian peninsula. In fact, the deserfication process was not clear till the past 4,000 years.
If I can recall, BBC had an old documentary that prooves the holy land was much greener during the Christ time than now.
Now to the question why it turned out desert lately, the answer can be provided by answering the question, how did they turn green before? the Monsoon that hits the Indian Subcontinent used to extend beyond the Arab Sea and reaches all the way to the Sahara. Some say it is caused by an increase of earth temperature that in return, strengthened the Monsoon. Others argue that Earth's axis was different in which all climate regions were shifted north.
I cannot give you exact example of the Sahara (but others here can probably). However, many sites in Saudi Arabia today depict a totally different climate and human activity than what it is now in presence. In Jubbah (a local village in Northern Saudi Arabia), the surrounding moutains depict drawings and carving of oxes, lions, hunters, and kings wearing ostrich feather. Not to mention the burried lakes and river beds under the sands of the Empty quarter. Similar findings are easy to find in the Saharah no doubt.
Check this webiste for more about the Sahara and the Arabian desert (includig maps of different epochs of both deserts):
http://www.esd.ornl.gov/projects/qen/nercAFRICA.html

I have long known the solution to turning the Sahara into a paradise:
1. Make a bunch of hydrogen fuel cell cars
2. Drive all over the Sahara so the exaust (water vapor) is released and forms rainclouds.
3. Continue driving until the Sahara is able to form its own rain clouds.

A more technical explanation is that the Sahara is caused by the fact that it lies in the centre of a high pressure air system caused by air descending from the equatorial zone.  What happens is that air near the equator is heated by the sun and flows north and south.  As it does so it cools at high elevations and begins to descend, coming to earth about 30 degrees north and south of the equator.  This descending air warms as it falls and dries up the region over which it flows thus creating the Sahara Desert.  This dry area extends around the world at this latitude.  If you look at a map you will see that Mexico has deserts in the same latitude as does the USA.  South of the equator there are corresponding deserts in Australia and Africa.
These large high pressure areas often feature regions areas of calm air known historically as the Horse Latitudes or doldrums.  They were featured in Colleridge's Rime of the Ancient Mariner. 

That is simple explantion.  If you want more detail consult a website on climatology.  This may help.
http://www.blueplanetbiomes.org/climate.htm