HistoAtlas is a free, open web atlas that maps cancer tissue morphology to molecular programs and patient survival across 21 cancer types. We analyzed 6,745 H&E-stained diagnostic slides from TCGA, extracted 38 quantitative histomic features (tumor morphology, immune infiltration, spatial organization, cell densities), and precomputed 10,000+ statistical associations linking tissue architecture to genes, pathways, mutations, immune subtypes, and clinical outcomes. 100% free. No login.