Rock Wall
Rock Wall
[Ponytail is gesturing towards Megan with her hand, while talking to her.] Ponytail: I live next to a wall of rock 20 miles thick.  There's no way around or over it. I'm trapped on this side forever. Ponytail: I study the stuff on the other side.
[Caption below the frame:] Mantle geology seems like the most frustrating field.

Mantle geologists study that part of the planet that's below the top " crust " of the planet.  The top layer of the planet, which is several dozen miles thick, is the only layer we've been able to explore, by digging tunnels , spelunking , etc. The only way to study the mantle and other inner layers of the earth are through non-visual, non-tactile, indirect methods, and by analyzing old samples of the mantle that have made their way to the surface.
In this comic, Ponytail , talking to Megan , is describing her job as a mantle geologist as that of living on one side of a thick wall that is, and likely always will be, impossible to get around, but she has to study what is on the other side of the wall.  In this case the wall is horizontal rather than vertical, the wall being the earth's crust, and makes a complete sphere, so the only way to get past the wall would be to go through.  It is theoretically possible to go through, but as of the comic's posting, humanity is far from doing so.  (The deepest hole dug as of at that time, as measured by true vertical depth , is the Kola Borehole , which only goes down to 12,262 metres out of the estimated 35,000 meters needed to get through at that location.)
In the title text Randall states that he doesn't trust mantle/core geologists. Because if they got the chance he believes they would not hesitate (even the duration of a heartbeat) to strip away Earth's crust to study the mantle or even worse the core directly. Of course if they only did this locally to look at the mantle it would not shatter the Earth although that local area may become a volcano. But if they actually peeled the entire outer layer away, we humans would have no place to live, as the mantle is really hot and would melt easily (producing magmas and therefore lavas when magma’s exposed to surface, see title text of 1405: Meteor to be more confused). However, after a while all these erupted lavas would solidify and become a new crust. Humanity needs to withstand just some millennia of active worldwide volcanism.
If they somehow exposed the core even locally something weird would be bound to happen. [ citation needed ]
But Randall is afraid that their craving to get around that 20 mile wall would prevent the researchers from even hesitating if they did get that chance. Fortunately, we can study planetary cores in the solar system without stripping Earth's surface by visiting an asteroid which is thought to be the exposed iron core of a protoplanet. The Psyche mission is scheduled to launch in 2022 and arrive at 16 Psyche in 2026.
This comics seems to be a spin-off from the previous comic 2057: Internal Monologues , where Randall tried to find some interesting monologues from scientist from different research fields. Maybe he did not find an internal monologue he liked for geologists, but ended up with this idea instead. Thinking about the core or mantle, lava and magma seems to be something Randall does a lot, and thus he must have some ideas about how a geologist would , as in 913: Core .
[Ponytail is gesturing towards Megan with her hand, while talking to her.] Ponytail: I live next to a wall of rock 20 miles thick.  There's no way around or over it. I'm trapped on this side forever. Ponytail: I study the stuff on the other side.
[Caption below the frame:] Mantle geology seems like the most frustrating field.
