This project aims to test the most fundamental concept to express our understanding of nature: quantum mechanics. It is an obvious fact that a quantum to classical transition happens for any large and massive object. This collapse is not explained by quantum mechanics itself. We aim to test if it can be understood by collapse models. If those are correct it could be a lead to formulate a more basic theory underlying to quantum mechanics. A specific model predicts collapse for a particle of mass of one million hydrogen atoms. One experimentalist (Hendrik Ulbricht (HU)) and two theoreticians (Angelo Bassi (AB), Tejinder P Singh (TPS)) will work together with two PhD students (with HU) and one post-doc (with AB) to define the experimental conditions to test collapse models and to perform the experiment. The experiment is a direct continuation of a FQXi large grant held by HU and we will here test the one million amu (atomic mass unit) prediction on the basis of knowledge gained with that grant. The collaboration with the theoreticians has been formed during a present JTF project, which resulted so far in a review article on the topic as well as in an essay written for an on-going FQXi competition. AB's work has been supported by a recent Italian grant. The project proposed here is a natural continuation of all that projects, with the new feature of a concrete experimental test. That will have a profound impact on our understanding of nature.