Lol, I have experimented with this for about 3 days and here is what I've come up with.
On a family server playing with my sister, she was building a railroad. I disguised a bunch of tnt (2x2) that extended 30 meters below the sea. I covered it with stone (pure/natural) and used a switch that used redstone to light it up. I blew a huge hole off of the railroad along with making the hole extend to bedrock level.

Another creative way, I build an obvious missile shell right in front of my house (on single player creative) made of Iron Blocks, Lapis Lazuli Blocks, and Red Wool. Inside of the missile shell it was filled with TNT (this took about 10 minutes to make). There was a space of 3x2 inside the missile shell and it extended up about 10 meters. I lit it off and it took my house along with the missile shell. (I lit it with flint and stone).

My sister was building a town and I made my way inside the town... build a "Mine" and instead of it being a regular mine, I had it about 5 meters under the town, and had a lock on the entrance (steel door with switch). I disguised it by putting a crafting table, bed, and furnace inside there. We were on creative mode btw. I put a hallway in and then I turned it....
AAAAAAAA
WWWWWA
AAAAAAAA (A representing air and W representing wall (Cobblestone).)

Then filled it with 20x20 TNT that extended about 4 blocks up.

I then built a "Community Center" in the town and filled it with tnt (or at least tried) I right-clicked by accident on the tnt too many times, it exploded, but what I did not expect happened. It set off all the tnt below the ground as well... my sister was really mad. Because guess what, the entire town is now gone! :)

There are a lot of creative ways to blow up maps and there are 3 examples listed above. Enjoy. Please post your creative ideas!