​Is this a bunker or grave? Found in my backyard. Please help identify!

This concrete slab has been in my backyard since I moved in but I never gave it much thought. My son and his friend wanted to uncover what we could and this is what we found. A concrete slab with maybe old bricks around 3/4 of the perimeter. Our house is old, so I’m not sure if maybe a grave site or an old bunker. Any help is appreciated!