The given tool for creating Maps in Source is Valves Hammer Editor.
I started creating Maps back in 2009 for Counter-Strike: Source.
Those were mostly Gameplay oriented Maps and didn't have much to offer if you were into graphical beauty.
In 2013 Counter-Strike: Global Offensive released and with it I started to create Maps again. This time they were still heavily Gameplay oriented to make them capable for competitive Play, but after enough testing and making sure they were balanced enough I started building the enviroment and creating beautiful Landscapes.
Since then I created 4 Maps from which 2 made it out of Betaphase and are polished enough to be included in a competitive game.