Mount Roraima is said to be the mountain that one in the Pixar movie "Up" is based on.
If I'm in Caracas, what's the simplest land-based way to reach Mount Roraima, and is it climbable?
The town you want to get to near the mountain is called Santa Elena de Uairén.
From Caracas there are direct buses (22 hours) or buses via Ciudad Bolivar (8h + 12h) or Puerto Ordaz (12h + 9h). Bus companies you can try are: Expresos Los Llanos, Expressos Caribe, Expressos San Cristobel and Expressos Occidante.
In Santa Elena you can get on a tour, a company with a good description of their tours online is Kamadac.
You can also continue by yourself, the Wikipedia page about Mount Roraima has a section 'Ascensions' which describes your options to get to the top.