What will happen if I stop paying my second mortgage?

The quick easy answer is  "If you don't pay......   they take it away". However there are a host of additional questions we have to address to give you the answers: Type of property – residence, non residence, commercial, rented acreage less than 2.5When were the 1st & 2d put on –                at original purchase             same lender or different lender             a later refi –  What did you do with the monies?Have you investigated short sales or bankruptcy? Look at A.R.S. § 33-729, 33 -814 G,  They could foreclose, sue on the note and forget the mortgage / deed of trust, file a credit ding, harrass you, sell the debt to a collector who will harrass you and may sue you ==or they could leave you alone.Even if they leave you alone, the debt is a lien on your home and does not go away.  If later your home goes up in value, the first is paid down, they can then come after you.You should get good advice from a competent Bankruptcy lawyer who deals with this all the time.