How did we see through Venus thick atmosphere?

The Venusian atmosphere supports opaque clouds of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible. Information about the topography has been obtained exclusively by radar imaging. Aside from carbon dioxide, the other main component is nitrogen

Explanation:

Because Venus has such a dense atmosphere which is some 100 times thicker than the earth's. Evidently, Venus is sufficiently close to the Sun that the little carbon dioxide it had in its early, Earth-like atmosphere caused the surface to warm up and leach out more carbon dioxide into the atmosphere.