This is a method I use very often to add detail to caverns or underground areas but also to outside areas (with water or dirt). The simple trick is, you just create a sector in form of some crackles (everyone knows how the look like, if not, try to draw "clef", the first 3 tries to produce this will be perfect crackles ;)) But as always images say more than thousand words, here we go:

Screenshot one shows a crackle from TNT : LE - Fury of Fire in which I used this detailing-method. I just drawed this crackle-sector with DoomBuilder, lowered the thing 8 units and changed the floortexture (and adjusted the lower textures from the parent sector to be also rock) and that's it. Furthermore you can change the crackle's light level, if the liquid you placed there is self-lighting. (buildtime 1-2 minutes)

Another great example for good crackles is shot number 2. You can see crackles everywhere, surfaces, where do old material broke away and the new "glowing" one appears. Imagine, I just used 2 different textures to create this rock, though, it looks high-detailed and like a hard effort (well, it really was ;)). The floor crackles are created as described above (almost look like tribals), the serious hard work was, where the ceiling-crackles move into the wall itself, where I had to use many small sectors but the build-concept is logical and easy to reproduce. (buildtime 20-30 minutes)

Summary: Crackles add  lot of spice and atmosphere to your outside and inside areas (cavern or technical), they are easy to produce and they look very natural (if you do 'em right ;)) and in combination with slopes, rock-textures and lava-stuff unbeatable regarding the work on caves! :)
Extra-Buildtime: "1-30 minutes" (depends on variation of the crackles)
Build-Difficulty: "Low - Medium"