This article shows how I created the photos for the Geology article.
Overview of the excavations used to create the geology photos
For reference (in the following description) North is at the top, and West to the left.
This world was created in "Creative" mode (create without limits, safely), with all four options (tree cover, terrain roughness, exotic terrain, and cave frequency) set to the default (center) positions.
First I found a good location -- a hill next to ice, that could be viewed from the side, marked and excavated to show a typical cross-section.
I marked the location of my "cross-section" with red blocks spaced at a distance of two blocks apart. This became the western edge of the excavated pit. For the rest of the process I did not touch (or even visit) any point to the left of the red dotted line. I stayed away from it as much as possible to avoid any disruption by Brown_Mobs (more on this below)
Some of the markers at the top of the hill are two blocks high because they could not be seen when they were only one block high. The yellow blocks were added much later, and show a straight line about 6 blocks below normal "ground level".
The photos in the Geology article were taken from the top of a ten-block "observation platform", here marked in purple at the far right side of this photo.
Most of the excavation is a gentle slope going down into the ground, clearing the line-of-sight for the hill cross section. This area is outlined with blue markers. It slopes downwards as you go towards the west (left in the photo). This took most of the time (about 10 hours of continuous gameplay, using the Notchium Drill III).
This type of hole turns out to be a very good way to trap astronauts. After about one hour I had 10 to 20 astronauts trapped in the bottom of the (still fairly narrow) hole.
An astronaut falls into the pit
When the Brown_Mobs encounter the astronauts, they explode, and these explosions might disrupt the pristine cross-section I was trying to expose. Therefore I had to draw the astronauts away from the edge (the red dotted-line) by making part of the hole slope downwards towards the east. This is the area outlined with green markers.
This part of the excavation had to be done quickly, and was successful -- I had a total of 4 brown-mob explosions during the excavation project. Two of these took place near the dotted light-blue line, which is where the astronauts were -- and far enough from the red/yellow lines to avoid defacing the excavated cross-section. The other two were farther east. Small patches of Wood (one near the light blue line) show where I filled in the damage.
The light blue (cyan) markers show where the pit is lowest. This slopes downwards towards the north. After getting into this trench, the astronauts tend to move further downhill, which takes them through the "gateway" outlined in light blue. Beyond this is a shaft that goes down about another 20 blocks, trapping them permanently.
At the bottom of the main pit, looking north
By the time I was done with the main excavation project and had my photos, a truly impressive number of astronauts had been collected. I eventually widened the bottom of the holding pen and added access ramps and observation platforms, just for fun.
The astronauts' cocktail party (giraffes drink for free)
Cross-section of the holding pen, with observation levels, connecting corridors and entry-exit ramps
After about 6 hours of gameplay, I stopped seeing new astronauts and mobs. More astronauts appeared only when I ventured into neighboring 128x128 superblocks (my article on the data format describes superblocks). I think there might be a limit to how many new NPCs get spawned, perhaps based on how many are already in a given section of the map.