Early one morning in April 2014, the water-damaged ceiling of the Jefferson Memorial portico finally surrendered to the elements, dropping a 5-foot-long, 3-foot-wide slab of limestone some 40 feet onto the marble floor below.

For months, the public didn't seem to notice. Then in October, a spokesman for the National Park Service -- which oversees the Jefferson Memorial and other monuments around Washington, D.C. -- confirmed that a faulty gutter system had allowed water to seep in from the structure's roof, eating away steel support beams until the ceiling could no longer hold the block.

It was the latest reminder that the National Mall is, both literally and figuratively, crumbling under the weight of its own decaying infrastructure.