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There is a lot of information on drying up a Mediterranean sized depression. However, there is not a lot of information on drowning such a large area or what sort of habitats could exist before the flood. eg before the Zanclean flood drowned the previously dry Mediterranean region and ended the Messianian Salinity Crisis.

In most scenarios the region would have to originally be mostly below sea level before the flood.

Most of my research around the Zanclean flood and other similar flood events have mentioned that it is typically in semi-arid and arid regions (ie previously dry) that these below sea-level depressions naturally dry up in. ie little rainfall or incoming river water. Which does make sense, as the water has to have gone somewhere...else.

I'm not focussed on how the depression would have become separated or rejoined to the larger ocean body. Just what sort of habitats are feasible before the flood, other than arid deserts?

Currently I am heavily contemplating drowning a large Mediterranean sized grassland and forest region, between 0 and 30 degrees north of the equator (equatorial region, lots of rainfall but also lots of evaporation). With a large dry desert above that.

Is it possible to have any other habitat aside from arid desert in such a large-area depression?

Affirmative answers should also address if and how this would be possible near the equator. ie Discounting a very large area of land subsidence, what natural features or processes would allow for an environment other than an expected lake or inland sea (BEFORE the flooding event)?

This is based off world but the planet has mostly earthlike processes and weather conditions.

Just incase, my planet does have a fair number of medium sized meteorites crashing down on it in the past, it can be very cratery in places. Back in prehistory, these impacts did do some substantial geological reorganising. I can increase/decrease this as necessary.

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    $\begingroup$ There is little literature because nobody has any good idea of what kind of conditions would prevail in such a deep depression. At present, the deepest depression on Earth is the Dead Sea valley, with water surface at 430 meters below sea level; as luck would have it, it is actually very close to the Mediterranean, so it may serve as a proxy: a hot dry lifeless desert. However, the average depth of the dry Mediterranean depression was 1,500 meters below sea level. Adiabatic heating caused by the high atmospheric pressure would make for very hot summers. $\endgroup$ – AlexP Apr 10 '18 at 20:11
  • $\begingroup$ I would agree with your instincts: "water has to have gone somewhere...else". Seas will dry up to become depressions in dry climate. If Mediterranean dropped a whole kilometer below the sea level and became very saline, despite inflows of Nile and other rivers, I think it's safe to assume that conditions around it were similar to modern day Dead and Aral sea areas. $\endgroup$ – Alexander Apr 10 '18 at 20:57
  • $\begingroup$ @AlexP, making the area not so deep could be part of an answer in how to get away with this? Could this be expanded into an answer, do you think, or is it still likely to be impossible. Especially in the equatorial region. $\endgroup$ – EveryBitHelps Apr 13 '18 at 6:26
  • $\begingroup$ Julian May, Saga of Pliocene Exile. $\endgroup$ – AlexP Apr 13 '18 at 7:09
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    $\begingroup$ I find this question quite hard to wrap my head around. You have a large endorrheic basin. OK. Now there are two possibilities: there is a a lot of water flowing into it, and in this case you get the Caspian Sea (water surface at −28 meters, average depth 211 meters) fed by the Volga; or there isn't, and in this case you get an arid or semi-arid depression possibly with an inland delta or a terminal lake, such as the Tarim Basin or the Helmand valley. $\endgroup$ – AlexP Apr 13 '18 at 8:17
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Yes, and the answer is nearby glaciation.

I checked the links you provided to refresh myself on the difficulties. Therefore, pardon me for doing more answering than explaining -- tell me if you think this works.

Non-arid means it has water and presumably plants to hold topsoil etc. Lower and presumably hotter than its environs means evaporation plays a key role. Salt does not evaporate with water and means that plants won't grow and soil won't stay. The only way to avoid salt without plants, water, and soil is going to be ridiculous (and skilled) winds, leaving only rocks, which is not what you are looking for. Water must be replenished and there must be a source which does not increase salinity.

Surrounding, or bordering on one side, and at higher altitude, a glacier can provide a fairly consistent source of fresh water -- consistent in decades or hundreds of years. That's long enough to build a civilization. You could even have the glacier irrigate most of your basin, and let the runoff with its accumulated continental salts collect in a deeper and hotter segment of the basin. Let "Death Valley" have its own salinity crisis -- now we have a locally complete water cycle.

Interestingly and poignantly (if unfairly) for SF terms, the salinity crisis could be a growing problem which must be confronted by the locals, only for them to be wiped out by a saltwater flood as the dam is breached.

When it rains, it pours. When it doesn't rain, it's worse.

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  • $\begingroup$ That is pretty darn ingenius! I don't know if it would work in an equatorial rainbelt area but could definitely work elsewhere! $\endgroup$ – EveryBitHelps Apr 24 '18 at 14:08
  • $\begingroup$ Put the himalayas on the equator and you will have glaciers. Look at Kilimanjaro. You want the equatorial rainbelt stuff in your civilized basin, right? Not necessarily where the glacier needs to go. You can have a great big shelf of glaciated highland. This will have fun weather effects. You can put the glaciated highlands upwind or downwind of the basin, so long as the water gets where it needs. $\endgroup$ – Haakon Dahl Apr 25 '18 at 4:29

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