Both craters have bull's-eye structures, with a pair of concentric walls
separating inner and outer sections of the crater floor.
Arata Isozaki and Yoko Ono collaborated on a labyrinth, which she calls a 'penal colony', comprising four concentric walls
of greenish ice blocks, stepping down from 10m to 2.
The die design is reported to boost productivity because tooling can run fixed-center or adjustable-center with concentric walls
at all speeds; the heating system is designed to ensure consistent heat throughout the crosshead; and consistent heat results in balanced polymer flow, thereby providing a stable and concentric finished product.
By the 1340s, Caffa was again a thriving city, heavily fortified within two concentric walls