This article presents the Zoomable Adjacency Matrix Explorer (ZAME), a visualization tool for exploring networks at a scale of millions of nodes and tens of millions of edges. ZAME presents an adjacency matrix graph representation aggregated at multiple scales. It allows analysts to explore a graph at many levels, zooming and panning with interactive performance from the most summary to the most detailed views. Several components work together to make this performance possible and the results meaningful. A ``pyramid'' of aggregated views paged on demand to OpenGL GPU shader programs supports smooth multiscale browsing in huge datasets. Efficient matrix ordering algorithms group related elements to make the views meaningful. Using ZAME, we can explore the entire French Wikipedia, over 500,000 articles and 6,000,000 links, with interactive performance on standard consumer-level computer hardware.