A very easy way to make a quick cave with the subdivide tool. First make an structure like that, this will be the base. Now we are going to divide the diferent zones, so we can subdivide. To subdivide in a correct way all edges must be touching other edges of another brush, so our base brush won't work. It should be something like that. Then we make it a displacement, but only the visible faces! If you subdivide all the faces of the cave it will crash! Only select the visible faces. It should look something like that, of course I changed the textures to fit the cave, now apply the alpha texturing and you will finish the cave. All credit for this tutorial goes to a great spanish mapper called Baxayaun, I have only translated it and I've made my own screenshots.