Magrathea is kind of a dump: the surface is dull-colored and empty—except for one big crater that seems to be scattered with blubber. Yeah, this is the crater that the falling whale made. Ew.
On the bright side, the crater opens up a path into the tunnels under the surface of the planet. As Zaphod explains, everyone lived underground in Magrathea.
Arthur asks why they lived underground: "Did the surface become too polluted or overpopulated?" (20.31).
No, explains Zaphod, it's just that no one liked the surface very much. Zing—take that, boring surface.
Zaphod leaves Arthur and Marvin to guard the surface, while he, Trillian, and Ford go to explore the tunnels, which means we're going to be using "meanwhile" a bunch.
While they explore the tunnels, Ford confronts Zaphod about how he found a totally lost planet. Or, as the British say, Spice Girls.
So Zaphod explains that life works out better for him if he doesn't think too hard about why he does things. But last night, he was worried about this very issue, so he ran some tests on his brains (remember, he has two of them since he has two heads). He discovered that part of his brain was sealed off and that someone had left his or her initials there.
The initials were—oh, this is totally like a horror story—"Z. B." (88), which happen to be Zaphod's own initials.
Before they can explore this fascinating insight, Zaphod and Ford get gassed and pass out.