Mining university rankings: Publication output and citation impact as their basis

World university rankings have become well-established tools that students, university managers, and policy makers read and use. Each ranking claims to have a unique methodology capable of measuring the 'quality' of universities. The purpose of this article is to analyze to which extent these different rankings measure the same phenomenon and what it is that they are measuring. For this, we selected a total of seven world university rankings and performed a principal component analysis. After ensuring that despite their methodological differences, they all come together to a single component; we hypothesized that bibliometric indicators could explain what is being measured. Our analyses show that ranking scores from whichever of the seven league tables under study can be explained by the number of publications and citations received by the institution. We conclude by discussing policy implications and opportunities on how a nuanced and responsible use of rankings can help decision-making at the institutional level.