Hi,
I bought a foreclosure property for my home, and the backyard is a mess. I'm trying to find out what this tree is, before I decide on whether to keep it or not. This tree is one of the nicest kept, but I don't know what it is. Can someone take a look and let me know?

Its fruit hangs on a branch that drops vertically, and the size is about a pingpong or goft ball. Inside the green shell, it's got another layer of very very hard & brown shell.

Many thank,

Sean