Not all students who could benefit from college apply. With novel data on over 1.2 million high schoolers, we show that nearly 25% start but never complete a college application. We use descriptive techniques, data visualizations, and fixed effects models to explore this population of college-interested "non-submitters" to observe application behaviors; document differences across individual, school, and community contexts; and identify factors most predictive of non-submission. We find large gaps by race/ethnicity, socioeconomic status, and education-career plans, as well as by school type and community features. We also find that early application tasks and engagement strongly predict non-submission. This study breaks ground for future research into this unexplored group and informs strategies to support those at risk of non-submission.