I do not understand the mechanics behind movement through solid earth. Please analyze my thoughts and questions.
If a creature with tremorsense 60 ft. is standing on solid ground, and there is a cave 30 feet below him, can the creature sense the location of that cave? Why? Tremorsense enables you to pinpoint the location of anything in contact with the earth. However, it is silent about whether you know information about the earth itself.
If a creature with tremorsense cannot detect the features of the earth around him, then that creature is blind for the purposes of moving through earth. This means that Earth Elementals must make an Acrobatics 10 check every round to keep moving through earth at full speed. (One might argue that Earth Glide says you move as easily as a fish through water, but water doesn't blind you and a blind fish is still a blind fish)
A creature with tremorsense can pinpoint a creature's location, but apparently that does not remove the penalties for blindness (the wording is similar to blindsense). Therefore, an earth elemental striking from within the earth, into a cavern, takes the usual penalties for being blind.
A druid which turns into an earth elemental does not explicitly gain elemental traits. Therefore, a druid presumably needs to breathe. If I understand correctly, then a druid must hold its breath whenever it is moving through solid earth, unless the druid has a way to bypass that restriction.
If a creature with Earthglide stays in the earth, adjacent to a cavern wall, then how is the creature affected by things dependent on sight? Can I occupy the earth, only poking my face out of it when I am making an attack (to negate blindness penalties) and moving (to keep an eye on features of the cavern), then duck my face back in and avoid gaze attacks?
Related to that, if I am in a square of earth, and somebody attacks that square of earth, do I take damage?
These are my personal answers to your questions. I take some pride on being aware of the details of the RAW and the spirit of the RAI, but in the end, just my opinions. But here goes anyway.
A creature with tremorsense should be able to sense the presence and basic dimensions of a cave, if only as a field of "solid" space. Solid as in blocks "line of sight" for tremorsense. As a side note, I think it is fair for tremorsense to give "basic" info about the surrounding earth. An iron fence post in the ground should come up as a "dense material", tree roots as a "semi-dense material", sand as a "soft material".
Granted, while I feel such information gathering is appropriate to Tremorsense, it is not supported by the rules: tremorsense is one of the "sense" categories of perception, along with lifesense, blindsense, and a few other unique "sense" senses. Blindsense gives a good description between what it and its superior cousin blindsight can do. Blindsense specifically states that visibilty affects movement normally, that is, a blindsense creature with no darkvision suffers all the penalties for moving around in the dark that a normal sighted character does. They just get to notice and pinpoint the square of any creatures.
Not sure I agree with that for blindsense, and I very much disagree for tremorsense on creatures like earth elementals and such, beings described as "one with the ground". But back to your questions.
You seem to have the majority of it already down, regarding the RAW on tremorsense. As far as the "popping your head in" question about caverns and such, I would run that as a sort of "blink" effect, granting both you and your target a 20% miss chance, as you are quite literally going from blind to not blind repeatedly. Much like blink, a readied action might be able to hit you without the miss chance, but you would still have cover for being in the earth, possibly improved cover.
As for gaze attacks, the rules are pretty clear on how they operate. You can look them in the eye, avert your gaze, or close your eyes entirely. Using your own eyelids, a hand, a blindfold, or six inches of stone doesn't matter, you hide your eyes from the creature, gaining immunity to its gaze but also becoming effectively blind.
Hope this helps in some way.
Thanks! That validates several of the things I thought about the subject.
A Druid's wildshape functions as polymorph except where it offers specific differences. That being said, under the polymorph description it mentions that if you gain a movement type, you gain the capacity to breathe that medium. It's said a little more eloquently, as climb doesn't give you the capacity to breathe a tree or rock.
By polymorph description I mean under the transmutation school in the magic section.
I've always viewed tremorsense as some form of subterranean echolocation. You can pick up the differences between several different materials in the ground, you can sense things in contact with the ground, and you can sense anything causing significant disturbance to the ground..
Thus the ruling would basically say that while underground you are not effectively blind. You would not receive a penalty for moving at your full burrow/earth glide speed. Also, you could indeed detect caves and the content of those caves should it be in contact with the ground.
Any flying creatures close enough to the ground to create a "thump" of air against it would be detected but would be more of a sporatic "ping" then anything.
Deciphering the information received could also be a difficult challenge to anyone that doesn't use that sense as a primary sense. Earth Elementals would do it naturally, as would anything that naturally has the sense.. However someone just picking it up through a feat, or someone getting it through some form of polymorph may have an "adjustment" period where they need to get used to seeing in that way.