Venus
Terraforming of Venus started roughly the same time as Mars, but due to the planets dense atmosphere and hellish temperatures is taking far longer. Floating platforms, using nuclear energy to power, were placed high in the planets atmosphere as growing beds for plant life allowing the atmosphere to be filtered. In the years sense large modern cities have been built floating above the surface as well, which is still to hot and arid for human life. Venus is known to be a dangerous place for colonists due to planet born illnesses.