So, I can't say for certain, but I am going to make an educated guess.
If I remember correctly, the devs have previously mentioned that Acacia is so dangerous to justify the amazing loot you can find in the Deep Forest. And let me tell you, there is no loot whatsoever to speak of in the forest itself, which makes me think that her den might be a mini dungeon that you can enter, like the entrance to The Sunken Caverns. If that is the case, then then entrance should be in the exact middle of one of the Deep Forest tiles; aka [-200,400], [-100,400], [0,400], [100,400] for the 4 top areas, or [-200,300], [-100,300], [0,300] for the 3 lower areas.
However, if I'm just completely wrong, and the Den is not actually a cave entrance or some such, then I have found at least one instance of ruins in the Deep Forest that has the inside area of 8x8 tiles, possibly matching the "8 meters deep" part of the description.
That's where I'll leave my investigation for now.