Brainstorm is a cooperative brain-computer interface (BCI) game that explores novel modes of user interaction enabled by web-based BCI systems. Participants transform a 3D brain—rendered as a 50k point cloud in WebGL—to display brainwave data at varying levels of scientific and artistic abstraction. All participants with a compatible EEG headset may remotely live stream data to the system. These participants cooperate to synchronize their brain activity in an experimental interactive experience. In parallel, all participants are encouraged to reflect on speculative applications as well as ethical, legal, and social concerns inspired by their experience. By hosting a series of events where diverse stakeholders engage with Brainstorm, we aim to promote the development of widely accessible brain-responsive experiences for the Web.