For decades seismologists have sought causal relationships between maximum earthquake sizes and other properties of subduction zones, with the underlying notion that some subduction zones may never produce a magnitude ∼9 or larger event. The 2004 Andaman Mw - 9.2 earthquake called into question such ideas. Given multicentury return times of the greatest earthquakes, ignorance of those return times and our very limited observation span, I suggest that we cannot yet make such determinations. Present evidence cannot rule out that any subduction zone may produce a magnitude 9 or larger earthquake. Based on theoretical recurrence times, I estimate that one to three M9 earthquakes should occur globally per century, and the past half century with five M9 events reflects temporal clustering and not the long-term average.