In this paper, we conduct a large-scale study on the crackability, correlation, and security of 145 million real world passwords, which were leaked from several popular Internet services and applications. To the best of our knowledge, this is the largest empirical study that has been conducted. Specifically, we first evaluate the crackability of 145 million real world passwords against 6+ state-of-the-art password cracking algorithms in multiple scenarios. Second, we examine the effectiveness and soundness of popular commercial password strength meters (e.g., Google, QQ) and the security impacts of username/email leakage on passwords. Finally, we discuss the implications of our results, analysis, and findings, which are expected to help both password users and system administrators to gain a deeper understanding of the vulnerability of real passwords against state-of-the-art password cracking algorithms, as well as to shed light on future password security research topics.