What is "sludge hole"

Sludge \Sludge\, n. [CF. Slush.]

1. Mud; mire; soft mud; slush.
   --Mortimer. Tennyson.

2. Small floating pieces of ice, or masses of saturated snow.
   --Kane.

3. (Mining) See Slime,

4. 4. Anything resembling mud or slush; as:

   1. A muddy or slimy deposit from sweage.

   2. Mud from a drill hole in boring.

   3. Muddy sediment in a steam boiler.

   4. Settling of cottonseed oil, used in making soap, etc.

   5. A residuum of crude paraffin-oil distillation.

      Sludge hole, the hand-hole, or manhole, in a steam boiler, by means of which sediment can be removed.

n. The hand-hole, or manhole, in a steam boiler, by means of which sediment can be removed.