[Answer] Which U.S. monument has a smaller replica hidden in a nearby manhole?

Step 1 : Introduction to the question "Which U.S. monument has a smaller replica hidden in a nearby manhole?"

...1. Statue of Liberty 2. Gateway Arch 3. Mount Rushmore 4. Washington Monument

Step 2 : Answer to the question "Which U.S. monument has a smaller replica hidden in a nearby manhole?"

Washington Monument - “Bench Mark A” is a 12-foot replica of the Washington Monument replica that was built in a manhole near the true monument. Surveyors use the secondary "mini-monument" as a Geodetic Control Point. Geodetic Control Points are a network of points around the nation that enable the National Geodetic Survey (NGS) to integrate all government maps. There are about a million such points across the U.S., most of which are metal rods or other stable unmovable objects. This particular control point was built to mirror its famous neighbor and has been used by surveyors since the 1880s. At first, the mini-monument was above ground but was later wrapped in a brick chimney and buried. It allows surveyors to measure things like earthquakes and, for instance, how deep the actual Washington Monument has sunk into the soil (6.2 centimeters over the past century).:

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