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The location isn't super obvious, however there are the slowing and sensor totems outside that give away the entrance. You'll definitely walk by near the cave's entrance as you level through Talador; it's located right next to the ramp that leads down from the Archenon Siegeyard (questing area where you take the shredder).
Inside the cave you'll find those same totems plus lasers from the ceiling, and if you can get past all of that you'll find the chest at the very end (under the beam of light).
Entering the red circle will slow you significantly, and entering the green circle will cause you to take substantial damage. You'll also need to dodge perpetually moving green laser beams coming from the ceiling. They move in a predictable circular fashion, so all you really need to master is the timing of when to walk by.
Walking through the red (slowing) circles is pretty much unavoidable. But it's harmless and as long as you can time walking through it with avoiding the ceiling laser, you'll be fine. The green (damage) circle should be avoided at all costs. They HURT. There's no reason to walk through them (unless you like to live dangerously and have an ability that negates all damage), as there's a clear path between each one.
If you're concerned about having to complete this walk all in one go, don't worry. There are spots throughout the path that are clear from both ceiling lasers and the damaging totems. You can take breaks and wait for just the right moment to keep moving. Timing is pretty key, and I found that following closely behind a ceiling laser, as it was making its circular pattern, worked for me. Movement enhancing abilities/buffs probably work wonders while slowed, but they're not absolutely necessary.
I did notice that sometimes the visual of the green circle, indicating the range of the damage totem, disappeared when I angled my camera a certain way. Thankfully, you can pause from time to time and double check to make sure you're staying clear of danger.
At the end of the cave you'll find a chest that you can loot. It looks like you can only loot it once; I haven't seen it respawn on this character yet.
I'm not sure if it's first come first serve, or if it's a phased area and every person that manages to successfully navigate the cave gets to loot their own chest (and receive their own pet). I'm definitely hoping it's the latter or else I might need to take a character, get past all of the lasers, and then camp her there while waiting for the chest to respawn.
As much as I enjoy dancing around lasers and possibly getting my face melted off, I don't think I want to make that walk more than once. :P
Plus if it's first come first serve, there might be an issue with someone camping the chest and preventing anyone else from looting the pet. Unless multiple people log out inside, competition for the chest will be hindered by the time it takes to actually reach it.
Some final notes on this pet, it's uncommon, soulbound and becomes common (white) quality when learned. You can cage it and have up to 3 in your pet journal. Best thing about this pet? Stonegrinder has some cute idle animations!