Hanka is a temperate agricultural Earth-like planet on which the SGC established Project 169, the Douglas McLean Memorial Observatory, under the command of John Smith of SG-7. The natives had welcomed the first contact by SG-1 and the efforts of the SGC to build the observatory to collect data on the accretion disk of a black hole during a local solar eclipse, but they also expressed the fear of their mythology that with the darkness of the eclipse would come the apocalypse.
When SG-1 returned three months later, they found that a biological infection had killed SG-7 and the entire population of 1432 people, except for Cassandra, a young girl who was rescued and brought back to Earth. The Goa'uld Nirrti had released the infection that contaminated the planet in order to hide her experiments to create an advanced human known as "hok'tar." The toxin was non-persistent, and four years later the air was clean when SG-1 returned to Hanka and found Nirrti's underground lab while searching for a cure for the retrovirus that had infected Cassandra as a part of those experiments. There are no remaining inhabitants on Hanka.
Cross Reference: Black Hole, Cassandra, Hazmat, Hok'tar, McLean Memorial Observatory, Nirrti
Episode Reference: Singularity, Rite of Passage