http://www.thegnomonworkshop.com/tutorials/natural-environment-creation-techniques

This six-hour workshop by Alessandro Cangelosi details his tips and techniques used to create large-scale natural environments, such as those used in visual effects. The workflow shows how Clarisse V4, as well as Houdini and Terragen, can be used to create impressive CG environments with realistic skies, atmospheric effects, and procedural assets.   While there are many workflows available to environment artists, Alessandro explains why his tools of choice are essential, such as Terragen for the creation of realistic skies, clouds, and for generating backplates for image-based lighting techniques. After introducing Terragen and showing how to take advantage of its atmospheric and cloud-rendering features, the lecture dives into Houdini, revealing how to tackle terrain modeling, build mountains, add details of all sizes, and how to use area masks, along with many other essential tasks. Houdini is also demonstrated for the creation of rocks and fog, detailing the workflow as a basic setup for every skill level to follow. Additionally, Alessandro covers exporting the rocks (Alembic) and clouds (VDB) — everything is generated procedurally.