Do you see a dome or a pit? Sometimes it is hard to tell! In this case, the answer is that we're looking at a pit, if the title didn't already give it away. Levity aside, we can tell this is a pit because we know what direction the sunlight is coming from and which side should be in shadow. This pit has formed on the south polar layered deposits. Why did it collapse? That is the real question to be answered. https://photojournal.jpl.nasa.gov/catalog/PIA24614