I don't remember much about the old 80's horror flick C.H.U.D., about mutants/monsters living beneath New York City and coming up to eat people, but it must have made an impression on me since it was the first thing I thought of when I saw this sewer cover tutorial.
I decided to offset the friction bumps by selecting a ring of bumps using the ellipse tool, cutting it, pasting it to a new layer, then using the transform tool to rotate it. I repeated that for the next layer in as well.
I decided to go with two holes along the edge rather than 4 holes in the top. I made them pretty much the same way except I:
Created an oval (on a new layer) at the edge with a stroke about 20px
Using the elliptical tool, I created a selection around the manhole over
Inverted the selection, and deleted it (removing the part of the ellipse that fell outside of the manhole cover).
Made a selection around the inside of the ellipse
Filled it with black
Keeping the selection in place so I couldn't paint outside the lines, I painted a bit around the edge with the granite texture using the rubber stamp tool. This makes it look like you can see the ridge the manhole cover is resting on.
Copied the 1st hole, rotated 180 degrees (using the transform tool) and positioned it on the opposite side.
I also added some blue filtering, adjusted levels, contrast and brightness to darken the image
Lastly, I added the blood spatters and bloody hand prints.