Mimas is the heavily cratered Saturnian moon often said to resemble the Death Star. Its resemblance comes mainly from Herschel, a vast impact crater that dominates one side of the moon.
Herschel is about 139 kilometers wide—roughly one-third of Mimas's mean diameter. The impact was so powerful that it nearly shattered the moon; the terrain on the opposite side appears disrupted by shock waves that traveled through Mimas's interior.
Mimas was discovered by William Herschel in 1789, and the crater was named after him. The moon's battered surface shows little evidence of recent geological activity, although modern measurements suggest that Mimas may contain a relatively young subsurface ocean beneath its icy shell.
The Death Star comparison is a visual nickname, not a claim that Mimas is artificial or hollow. Another common mix-up is Saturn's Cassini Division: Mimas's gravity helps maintain that major gap in the rings through orbital resonance, but the gap is not a feature on Mimas itself.