The alien in the garden

Anybody know what the heck this is? (I show it with a tomato for purposes of scale.)
Never seen one before, and it's growing in my garden (along with many others) on a huge vine.

First I thought it was a zucchini, because it has leaves much like our other zucchini plants, but the "fruit" is not long and elegant like a zucchini, but kind of football-shaped. Then I thought it might be a mutant cucumber or melon, but when I cut it open it didn't have crisp cucumber or melon flesh but was kind of soft and squashy-looking.
I Googled images of every imaginable squash and nothing looks like this, our garden alien.
I'm tempted to cook it up and see if it tastes like squash, but I don't dare risk dying of food poisoning before I finish my avowed year of blogging. So I turn to you.
Any ideas?
And while you're at it, any thoughts on how to keep the @##$%% chipmunks from eating my tomatoes?
I'll share the first canned tomato sauce with whomever can help, honest.