Bruno Willis

Use the Sticky Dirt mod: https://mods.vintagestory.at/stickydirt  It is fantastic, makes sideways soil instability make sense. It ties soil instability to vegetation cover. Plant roots make things more stable, essentially. You'll still have blocks slip out from under you if you're climbing steep cliffs, and dirt still caves in when you're digging it, but it stops whole hillsides collapsing because a rabbit walked by. 
I never play without it, and without enabling soil sideways instability. I wish sticky dirt was part of the vanilla game. 

Welcome to the forums. That cog is your temporal stability. If it's rolling towards the right, you are stable. If it's rolling to the left, you are getting less temporally stable.
Be careful when it turns to the left, it means either you are in an unstable surface block, you've gone too deep, or you're in the middle of a nasty temporal storm. Either way, as the teal glow gets lower on the cog you will start hearing strange sounds, getting visual glitches, and eventually spawning rust foes all around you. 
To increase your stability, either hold a temporal gear in your off hand and use a knife (I think implanting the new gear inside yourself...) or kill some rust foes, or get out of there. The last option is usually the best one, when possible.
Good luck out there in that broken world!

I've found deep deposits, and they're barely larger than surface deposits. If you're good with a pro-pick, they're a decent thing to look for. They seem to generate kind of high up. I've only bothered when I've got a Galena note while prospecting for something else, and it's usually been kind of difficult to actually hit the deposit, because they're so small. Persistence and having plenty of pro-pick durability is key.