What is under the back interior of a ford explorer?

Looking into a cargo vehicle of some sort, but with 4X4. A minivan is a great choice, but many do not have 4X4 option. So I'm looking into a larger Ford Explorer. Looking into the back area, it may look like there's some wasted space. As the photo says, what is under this?

Now I mean, by taking it apart completely, stripping it down to metal, and sealing it up. But what is under the fabric and plastic flat area. What is under the back interior of a ford explorer

Spare tire

No

Spare tire and fuel tank.
Go underneath and look.