iPhone 3GS: Swollen battery.

Good morning everyone, i have a big problem with my old iPhone 3GS. I think that the battery is literally going to explode, it's really swollen. What can i do? I bought my iPhone in 2009, it was perfect, no scratches, never dropped it, it worked well. I haven't use it for 3 years and now that i need it, the screen is "lifting" from the body of the phone. I think this isn't a really safe thing. I can't contact Apple online because my warranty has expired. But i want to know what happened to the phone. Please, can someone explain to me what happened?

I am from Italy, i tried to contact an Apple reseller here, near my town, they said to me to keep the phone turned off and nothing will happen. I don't believe that "nothing will happen".