Research infrastructures help scientists conduct research; thus, researchers want to have them, such as big machines or databases, which is why policymakers and funders keenly invest in infrastructure building. However, this paper challenges these taken-for-granted assumptions: in some cases, research infrastructure building disturbs research work; researchers do not want the infrastructure; policymakers and funders are not happy with the progress. Still, we will probably have another research infrastructure anyway. Therefore, we should understand better underlying challenges in building research infrastructures. To this purpose, this paper examines the Human Brain Project (HBP), the EC’s ongoing largescale project intended to build a digital research platform for brain-related sciences, which also is part of the European Strategy Forum on Research Infrastructures. Since the 1990s, inspired by the Human Genome Project, brain researchers have made efforts to build a large research platform, notably the US Decade of the Brain, though without concrete results due to the complexity of brain data. Then, the HBP might be considered as the continuation of the neuroscientific community’s desire. However, from its launch in 2013, the project has suffered from internal and external contestation over its scientific orientation, necessity, and feasibility. Based on interviews (n=60, approx.) and direct observations, this paper focuses on (1) the convergence of the neuroscientific community’s long-awaited desire for a large platform and the EC’s recent Research Infrastructure politics, which enabled the HBP’s launch; (2) the political, epistemic, and technological challenges in building such a research infrastructure. It also highlights the HBP’s ambiguous status between a research network infrastructure comprising around 600 scientists and an engineering project intended to build the digital platform, which has aggravated the challenges. The paper concludes with remarks on the tensions between politics striving for “something big,” technoscientific promissory projects, and the implementation of those “big projects.”