Hahaha - great concept, and definitely one to get us thinking - oh and I love how you've drawn the clouds ^_^
that's cool, it reminds me of Astro Boy's Metro City and how it looked like a city underneath a city. That'd be cool if you designed characters for that place too, fish people or something.
Like the last one-- a great concept. I would like to see alittle more finish to the peice. The idea is solid maybe look to Matte painting techniques to push the cityscape by adding texture overlays and alittle image mapipulation. Also why not push the city more right now it looks about the same mass as the tip of the iceberg why not make it larger and more grand try to really play off that idea of just seeing the tip with a massive underworld submerged. So far your peices have been very graphic as far as the style. I would love to see alittle more attenetion to the details it doesnt have to be super rendered every where but that city is begging for it!