What happened to the Northern hemisphere of Mars?

Question

http://i58.photobucket.com/albums/g261/mcrib_is_back/mars_topography.jpg

The Northern hemisphere is way below the average elevation for the planet. Also, it seems a lot smoother than that pockmarked southern hemisphere. The North pole of Mars also sits on top of a dome that's almost 3 kilometers above the surrounding area.

http://i58.photobucket.com/albums/g261/mcrib_is_back/Mars_nth_pole.jpg

To help you give me a better answer, here are a few more pictures of Martian phenomena:

Glass tubes: http://i43.photobucket.com/albums/e397/Bigpappadiaz/slide25.jpg

Craters centered on other craters:
http://i58.photobucket.com/albums/g261/mcrib_is_back/050606omcaldera.jpg

Craters with mounds raised in the middle, that resemble results of electric arcs striking clay anodes in lab experiments (note the how the mud "sweats" away from the crater):
http://i58.photobucket.com/albums/g261/mcrib_is_back/crater2.jpg

and the "collapsed lava tubes" of Ascraeus Mons (plasma scars):

http://s43.photobucket.com/albums/e397/Bigpappadiaz/?action=view¤t=051111lavachannels.gif

The first answer was better than the second. Sorry second dude, that just ain't the way it is.

Answer 1

~Yo, Dude. The remark to answerer #2 oughta get you a lot more responses. If you're such a know it all, why post the question.

Answer 2

They believe that mars used to hold a great ocean - the ocean covered the northern hemisphere of the planet while the southern most held vast mountain ranges - supporting huge volcanoes - Since the planet slowly died losing its vulanism the gravity of the planet faded away and the oceans and atmosphere disipated - the smooth flat section you see today are the remnants of a huge ocean smoothing out the terrain and the huge dormant volcanoes and old craters are seen today on the southern part of the hemisphere.

Answer 3

Yea, that was one hell of a party, good times, good times.