Explaining is a vital spontaneous process for human beings, as through explanations we make sense of the surrounding phenomena. While the role and structure of explanations have been explored in each science, technology, engineering and mathematics (STEM) area, little is still known regarding what counts as a good explanation for a STEM problem. This study seeks to address this void by examining the nature of 82 explanations of a STEM challenge produced by 9th grade students (years 14-16) who participated in a learning sequence designed with an integrated STEM education perspective (I-STEM). We aim to find out which types of explanations are produced by 9th grade students when engaged in an I-STEM learning sequence, as a strategy to establish/identify the core features of a good I-STEM explanation. As a subsidiary aim, we intend to propose an analytical framework that enables the characterization and understanding of the students' explanations to an integrated STEM problem, based on the combination of a theoretical view on I-STEM explanations and the qualitative analysis of explanations produced by students. Results show that a majority of the students produced STEM explanations, either descriptive or relational, that failed to demonstrate knowledge or procedures from the four STEM areas. About a quarter of the solutions entailed I-STEM explanations, thus recognizing the role of knowledge or procedures of each area, concurring to obtain the solution, from the creation of the artefact to the understanding of the underlying law. We further propose a conceptualization of a good I-STEM explanation as involving the mobilization of conceptual knowledge and processes from all STEM areas in an explicit and intentional way to present the solution devised to the STEM challenge.