Here's why:

* Thickly Clouded: Venus has a dense, highly reflective atmosphere of carbon dioxide, making it perpetually shrouded in thick clouds. This makes it difficult to see its surface from space using visible light.

* Radar Mapping: Radar can penetrate these clouds and map the surface by bouncing radio waves off the terrain.

* Varied Topography: Radar mapping has revealed that Venus's surface is surprisingly diverse, with vast plains, towering highlands, and numerous volcanic structures, including shield volcanoes, lava flows, and even calderas.

Let me know if you have any other planetary mysteries!