Using NASA’s James Webb Space Telescope, astronomers uncovered a mysterious feature within the remnant, nicknamed the “Green Monster,” alongside a puzzling network of ejecta filaments forming a web of oxygen-rich material. When combined with X-rays from NASA’s Chandra X-ray Observatory, the data helped astronomers shed light on the origin of the Green Monster and revealed new insights into the explosion that created Cas A about 340 years ago, from Earth’s perspective.