Which planet's atmosphere is mostly carbon dioxide with sulfuric acid clouds?

Venus - about 96 per cent carbon dioxide at ninety times Earth's surface pressure, which is what you would feel a kilometre underwater, under permanent cloud made of sulphuric acid droplets. The surface is around 465 degrees Celsius everywhere, day and night, pole and equator.

The Soviet Venera programme is one of the more extraordinary engineering efforts nobody remembers. Between 1970 and 1982 they landed several probes built like bathyspheres, and the best of them survived just over two hours before being cooked and crushed. They returned the only photographs ever taken from the surface of another planet's ground apart from Mars - flat plates of rock under an orange sky, with the lander's own dropped lens cap visible in one frame.

The interest now is that Venus may not have started this way. Models suggest it could have held liquid oceans for a billion years or more before a runaway greenhouse took over - water vapour trapping heat, driving more evaporation, until the oceans were gone and the hydrogen escaped to space. It is the nearest available experiment in what a thick carbon dioxide atmosphere does to a rocky planet, which is why several missions are now heading there after thirty years of neglect.