What is that smell?

Surely I left a dirty diaper in the car because everyday it smelled worse and worse! I looked everywhere for this dirty diaper to no avail. The stench was horrible! After a week and a half of stench, Chris took the car to work. When putting packages in the trunk, he noticed a dead decaying frog caught in the trunk door. He sweetly took the frog off and put it in our driveway for us to see. Nasty! It's as hard as a rock!