Today in a mailing list thread discussing the property of circular manhole covers that they cannot fall through thier respective manholes (assuming the manhole has a lip of nonzero width):

They said “manhole cover” which I assumed meant a 3D object, made of steel or aluminium, impressed with a non-slip pattern and a city engineering sigil, and with a slight upwards dome shape to resist compression. Though I believe we’re supposed to call them “personholes” in this day and age of political correctness.
-Matt