How does one delete objects underground?

Hi.
Please keep in mind that I'm a 2 week old noobe.

I decided to redesign my leased property, so I deleted all unwanted prims, kept what I want in my inventory to start from scratch.

I took a look at my land statistics and showed 98 Prims used.
I already deleted two items (returned to owner) found on my property- suspecting more I flew as high as possible with the page up button.
Sure enough there was a type of futuristic AIRPLANE hovering up above. So I sent that back to it's original owner. My prim count is now 20used out of 937.

I look at my mini map and it shows two blocks still on my property: One is black and the other is light blue. (I have kept the default colors in my preferences)

I am thinking there are still items possibly above, and below.
I'm pretty sure my first attempts at building I myself may have left something underground. I can mouse look while hovering a hundred meters above one object, and I can see two of those flashing animations like one would use on jewelry, but again it's underneath.

Can anyone tell me how to find/delete these items?

Thanks in advance.

You can turn on the Advanced menu with Crtl-Alt-D, then make the ground disappear with Advanced->Rendering->Types->SurfacePatch. the same will restore it when you're done looking.

Or, you can alt-click on the ground, and use Ctrl-Alt-Shift-downarrow to peek under there. Alt-arrow keys will let you move the camera around once you get down there.

Ctrl-Alt-T might be useful here too, to highlight transparent stuff. Also, play with the Beacons stuff under the View menu, then can sometimes help to locate where things are hiding (though I have to admit I don't find that as useful as it could be since the lines are drawn oddly).

Keyboard shortcut is CTRL+ALT+SHIFT+5

the light blue object belongs to you.

Or look at what the menu says

Thanks for the responses. I was able to delete everything (and send it back) but there is still a structure (The dark one on the map) which is really, really deep and I can't seem to select it. Any ideas? I'm standing on my land which is now a clear prim, directly over the object(s).

Please disregard: I went to the same advanced menu and unchecked "Ground" and was able to send that object back to it's owner. I'm now 13 prims out of 937, the map is clear so I'm good to go!

Thank you all, you've saved my day!

-Zerock