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Message: <blockquote data-quote="Nyeshet" data-source="post: 2500434" data-attributes="member: 18363">Air pressure should not be a major concern until around 4+ miles deep (perhaps a little deeper). As for the flora, consider an inversion of the classic altitude / latitude presentation (using a mountain in the tropics, showing slow diffusion from tropic to desciduous to boreal forest, then to tundra, and finally to ice). Assuming the same (look <a href="http://curriculum.calstatela.edu/courses/builders/lessons/less/les3/lat-alt.html" target="_blank">here</a> for an example), we find that: [code]feet      temp*     biome0k - 4k   30 - 42   ice, frozen waste4k - 8k   42 - 54   tundra8k -12k   54 - 66   boreal forest12k-16k   66 - 78   desciduous forest16k-20k   78 - 90   tropical forest* This is the average temperature.[/code]Of course, by the time you are 20k deep the air pressure is becoming a bit of an issue - much like climbing 20k feet will be a bit of an issue. (It is called the 'Dead Zone' by climbers for a reason.) Actually, by the time you are 8 - 12 ft deep you face a bit of a problem, as the air pressure is a bit more than double the norm (for sea level). Of course, if you start at a higher altitude then descend, then the resultant air pressure would be about normal (or at least less than double normal). I would suggest starting around the arctic circle in an area perhaps 4k to 8k ft above sea level. The region would normally be tundra with trees not too far to the south (several days journey on foot, perhaps). There create a valley about 10 - 14k ft deep (~6k ft below sea level, so only about 1.25x to 1.5x sea level air pressure). Now we have a mix of boreal / desciduous - mostly desciduous in the deeper regions and mostly boreal in the 'higher' regions. The rain from snow fall that passes over the valley surface combined with the steam from geyseurs, hot springs, mud flats, etc give the area a slightly more humid feel, allowing for more plantlife than typical (as tundra biomes are not exactly known for their percipitation). The clouds that rise from intra-valley evaporation do not rise high enough to escape the valley lip, so most of the moisture within does not leave. Some does, but it is eventually replaced with snow fall (that becomes rain as it falls into the depths of the valley). The bottom of the valley will be difficult to breath if one is not used to such. I would use the rules for altitude adjustment in the DMG (or SRD) in reverse as one descends (or ascends, if they are attempting to leave). Once used to it they would have to adjust again upon leaving. Treat the valley floor as a forest prone to occasional mists (esp during transitional periods, such as morning / evening and autumn / spring) Some flat treeless areas would exist, but such would often have mudflats, geyseurs, etc. Hot springs and a few seasonal streams (more likely, waterfalls) from the mountains surrounding the valley lip would be the source of the streams and (very few) minor rivers present - which eventually form lakes. These lakes would partially dry up during the autumn / winter (when meltwater from above is not sliding into the valley) and fill during the spring / autumn. This shift could lead to agriculture along these streams / minor rivers and around the lakes they eventually form. Another question to consider: do you use the concept of ambiant magic in your world? If so, how does such a rift (or collapsed supervolcano) affect local magic? If based on ley lines, such a rift (or sv) might be particularly potent in magic, suggesting magical creatures (and beings and classes) being more common.</blockquote>

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