This happened to be my first encounter of a slime. It was Huge!

I was mining away, when i heard this clicking/slapping/clunking sound. It was really weird. I thought it was a dispenser or something going off.... but When i finally got the guts to go check it out, through this wall, there was NOTHING there!! and so i walked around, testing where the sound was loudest, then i got my tnt, and blew it up XD But it took two blows, before i could see this wonderful green thing looking at me.

It was stuck. STUCK, bouncing up and down, only to be trapped by stone. So i got rid of all the blocks around it, and in the process, i accidently hit it, and it went bloody rampant! So i ended up hitting it, so i have three now, but they're all locked up :)