Warp wrote:
>   You left out the fact that a round cover will not fall into the manhole
> no matter how you might screw up opening it.

And what everyone so far has missed is that roads get repaved. The 
manhole screws into the vault at the bottom (so you can make it flush 
with the surface of the road) and has threads on the outside. When you 
repave the road, the first thing you do is give all the manholes an 
extra crank or two to lift them up an inch or so, so they stay flush 
with the surface of the road after you add more asphalt.

Plus, of course, it takes less metal to make a round cover to cover a 
certain opening than any other shape, which is important when you're 
planning to place millions of manholes and calculating how much your raw 
materials will cost.

-- 
Darren New / San Diego, CA, USA (PST)
  Helpful housekeeping hints:
   Check your feather pillows for holes
    before putting them in the washing machine.

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