Architecturally, which is the only arched structure in the world that can be built without a supporting struct

Answer 1

The Roman Arch--the keystone in the middle keeps the tension equal on both sides and supports the weight above.

Innovation started in the first century BC, with the widespread use of concrete, a stronger and readily available substitute for stone. Brick-faced concrete quickly supplanted marble as the primary building material and more daring buildings soon followed, with great piers supporting broad arches and domes rather than dense lines of columns suspending flat architraves. The freedom of concrete also inspired the colonnade screen, a row of purely decorative columns in front of a load-bearing wall. In smaller-scale architecture, concrete's strength freed the floor plan from rectangular cells to a more free-flowing environment.

Answer 2

In St. Louis, the arch over the Mississippi River.

Answer 3

An arch. It not only supports itself, but actually becomes stronger the more weight you put on it.

Answer 4

a vault?