What vegetable is growing in my garden?

We had a vine overtake our garden a month or two ago. A vegetable of some type is growing. One single plant has now almost taken over a 12' X 12' garden. A squash or zucchini is what we think it is. It is striped like a watermelon, but long like a zucchini. It is thinner at the top, going to a more bulb shape at the bottom. Any ideas?