There are two unmarked stone towers on the western slope of the Capitol Grounds. Overshadowed by the Capitol dome and downplayed by blocking shrubbery, most people walk by without noticing them.
These are the end caps for two massive air ducts leading 400 feet up into the belly of the Capitol. You can see the 15-foot-wide passageways marked here in a 1961 map. The asymmetry of their layout corresponds to differences in elevation on the House and Senate side of the hill.
The air ducts date to 1928 and were part of an engineering effort to install air conditioning in the Capitol. Summer in Washington is notoriously muggy, and this unpleasantness was amplified in the crowded and windowless legislative chambers. Some Members of Congress took this really, really seriously.
The House of Representatives Historian notes:
The Senate watched this with envy and quickly moved to install their own AC system. Posted notices served to reassure elderly Senators that the cool air was nothing to be frightened of.
The system worked by drawing in air from the towers on the Western Front park. A fan the size of a ship’s propeller blasted the air through a dense radiator-like array of pipes containing cold water. Then it passed through the humidifier room where a spray of water restored an appropriate level of moisture. Thence, up into the Senate and House, where it entered the chambers through grates in the floors.
The Carrier Corporation—perhaps excessively—predicted that the AC would have “a profound effect upon our governmental system! Congress may voluntarily remain in session throughout the summer, in order that our Congressmen may be protected from the intolerable discomforts and dangers of the ordinary outdoor weather!”
Those air duct towers deserve a plaque befitting their impact on our great democracy.