There are white dots in the higher quality render, compared to a cleaner low quality render.
- Low quality is the default setting in Terragen
- High quality
Render: micro-poly detail 0.8
Render: Anti-Aliasing 8
Atmosphere: number of samples 64
Sun: do soft shadow
This looks like a displacement issue to me. Run through your fractals and switch off all displacements that aren't contributing to anything. By default all power fractals have displacement on. :o Equally they all have minimum scale set to the equivalent of 1cm so if you have lots in your scene pushing different directions you might get holes like this, particularly at higher details.
If you want to leave things as they are, try playing with the Displacement Tolerance slider at the bottom of the planet panel. Bring that down to say 0.25. Your render will take forever and day that way though. Try the first thing first. ;)
Alex is right on most accounts, except on the lowering to 0.25 of the displacement tolerance. For less holes you should raise it by 10% or more, depending on the occurrence of holes. If 10% isn't enough 20% or more might be, but it renders slower indeed.
Additionally, holes can be created from chaotic PFs with too high of spikes compared to the offset of the terrain where spike crevices push into the offset terrain. They basically tear the terrain. This is almost a religious fix for me on scenes with vertical displacement on offset terrain. Like towers.