  Recent advancements in 3D dust mapping have transformed our understanding of
the Milky Way's local interstellar medium, enabling us to explore its structure
in three spatial dimensions for the first time. In this Letter, we use the most
recent 3D dust map by Edenhofer et al. to study the well-known Chameleon,
Musca, and Coalsack cloud complexes, located about 200 pc from the Sun. We find
that these three complexes are not isolated but rather connect to form a
surprisingly well-defined half-ring, constituting a single C-shaped cloud with
a radius of about 50 pc, a thickness of about 45 pc, and a total mass of about
$5 \times 10^{4} \mathrm{M}_{\odot}$, or $9 \times 10^{4} \mathrm{M}_{\odot}$
if including everything in the vicinity of the C-shaped cloud. Despite the
absence of an evident feedback source at its center, the dynamics of young
stellar clusters associated with the C structure suggest that a single
supernova explosion about 4 Myr to 10 Myr ago likely shaped this structure. Our
findings support a single origin story for these cloud complexes, suggesting
that they were formed by feedback-driven gas compression, and offer new
insights into the processes that govern the birth of star-forming clouds in
feedback-dominated regions, such as the Scorpius-Centaurus association.
