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This is Siwa.中英文稿
这是锡瓦。
It's a crowed place with bustling markets and around 33,000 people.
这是个拥挤的地方，有着繁忙的集市和大约33000人口。
Here you can find farms which grow things like dates and olives and trees grow as far as the eye can see.
在这里你会发现种植海枣和橄榄等作物的农场，目所能及之处皆为树木。
Siwa has been permanently inhabited since the 10th millenium BCE or for roughly 12 thousand years.
自公元前10世纪，或是约12000年以来，锡瓦一直有人居住。
The city contains several archaeological sites and is even the site of an oracle temple for the Egyptian deity Amun.
这座城市有数个考古遗址，埃及神祇阿蒙的神庙遗址甚至就在此地。
What's all about Siwa is that it's located here, completely isolated in the Egyptian portion of the Sahara desert.
外界只知道锡瓦坐落在此，完全隔绝在埃及境内的撒哈拉沙漠中。
The nearest big city is 460 kilometres away in Alexandria, and the ocean at its closest is 260 kilometres away.
离它最近的大城市是亚历山大市远在460千米外，最近的海洋则在260千米外。
This deep in the desert, Siwa receives only 9 inches of rainfall a year, and temperatures in the summer have reached a blistering 118.8°F.
锡瓦地处沙漠深处年降雨量不过9英寸，而夏季的气温已经高达118.8°F。
Just outside the limits of Siwa, tremendous and imposing sand dunes linger, constantly threatening to advance.
巨大而壮观的沙丘，就徘徊在锡瓦的边界外，时刻威胁着前进。
So how can a place so deep in the desert be so rich with people, vegetation and history?
那么一个如此深入沙漠的地方怎会居民众多，植被茂盛，历史丰富呢？
Well, as it turns out, Siwa isn't just a city, but rather an oasis, a fertile patch, and otherwise endlessly barren land.
原来锡瓦不止是一座城，还是一片绿洲一片沃土，在它之外则是无垠的贫瘠之地。
But what exactly is an oasis and how do they form?
绿洲究竟是什么？ 又是如何形成？
In the past, the desert oases have served a crucial role in trade and travel as hospitable stopping points wedged between vast expanses of sand and stone.
过去沙漠中的绿洲是舒适的停靠点，它们嵌在广阔的沙漠中，对旅游和贸易起着至关重要的作用。
In places like Siwa, merchants and traders could rest, resupply and most importantly refill the waterskins.
在锡瓦这样的地方，来往客商可以停下休息，补足物资，最重要的是重新装满水袋。
Most oases will have fresh water springs or ponds above the surface open to the air, and this is what allows both people and plants to inhabit the area.
大部分绿洲都有位于地表的，露天淡水泉或淡水池塘，这是人和植物能够在此栖息的保障。
But a look from above reveals no river or even stream feeding into these ponds.
但从空中俯瞰、不见河流，甚至不见小溪流入这些池塘。
And as I mentioned earlier, areas like this received little to no rainfall annually.
我之前提起过，这样的地区年降雨量很少，几乎为零。
So where does this life-giving water come from then?
那么，这生命之水又如何而来呢？
Let's keep using Siwa as an example.
依旧以锡瓦为例。
Well, this land may receive just a few inches of rainfall a year, over a big enough area, a couple inches can add up to a huge amount of water.
这片土地的年降雨量或许不过几英寸，但在一片足够大的区域，几英寸降雨量相加可得大量的水。
And you'll typically find an oasis at a low point in the desert.
你会发现绿洲通常位于沙漠的低洼处。
A look at a elevation map shows the Siwa oasis to be right in the middle of a depression, beside the larger Qatar Depression.
地形图显示，锡瓦位于大洼地的正中间，在更大的卡塔尔洼地旁。
In fact, most of the Siwa oases lies at roughly 19 metres below sea level.
实际上，锡瓦绿洲大部分低于海平面约19米。
So, the Siwa and other oases are essentially funnels for all water from surrounding areas to flow into.
因此，锡瓦和其它绿洲实为漏斗，周边地区所有的水都会流入那里。
But again, there aren't any rivers or streams on the surface to bring water to these places, so how does it get there?
但需要重申，地表没有任何河流或溪流，将水带入这些绿洲，那么水是如何流入？
Well, beneath the surface of all land, there is an area where rainwater collects.
在所有土地的表层之下，有一个收集雨水的区域。
We call this an "aquifer".
我们称之为“地下蓄水层”。
If it rains a lot, the average of the aquifer called "the water table" will rise.
如果降雨量大，蓄水层的平均水位，即“地下水位”会上升。
If it rains very little, the water table will drop.
如果降雨量很小，水位会下降。
It's by taking advantage of underground aquifers that things like wells and irrigation systems work.
正是利用了地下蓄水层，水井和灌溉系统等工程才能运行。
Naturally, where the elevation of the earth is higher, so to will the water table be.
地面海拔越高的地方，地下水位也自然越高。
So if you have areas of high elevation like towering sand dunes or hills, directly next to areas of low elevation like a depression, you can get spots like this where the water table nears or even exceeds the surface.
因此，如果是高海拔地区，比如高耸的沙丘或山丘，紧挨着低海拔地区，如洼地、便能看到这样的地方，那里的地下水位临近地表，甚至超过地表。
At these places, water will see from the ground and all into bodies of water.
这些地方的水会露出地面，全部流入水域。
Where there is water, there is life.
有水之处即有生命。
And while most desert plants are short and dense, under the right conditions, they too can grow to be tall and lush.
尽管大多数沙漠植物矮小密集，但如果条件合适，它们也能长得高大茂密。
And there you have it. An oasis is formed.
正如你所见，绿洲形成了。
It helps to know also that the water table can be further altered by impermeable layers in the soil, such as rocks or dense layers of clay called "hardpans".
这还帮助我们了解土壤中的不透水层，例如岩石或被称为“硬质地层”的致密层，也能进一步改变地下水位。
Because water cannot pass through these substances, its only choice is to move the water table upwards,
由于水无法穿过这些物质，只能抬高地下水位，
and this can be another factor in bringing water to the surface, even if there is no depression.
这样即使没有洼地，也能将水带到地表，这可能就是另一个原因。
And that's all an oasis is.
以上就是绿洲的形成。
Light rains from a large enough area can flow through an underground aquifer hold by gravity, forming basically an underground river.
在大面积区域下的小雨，由于重力流过地下蓄水层，大体形成一条地下河。
Eventually, plants and animals will find these upwellings water and grow to create a miniature ecosystem, a microcosm of rich life, and in otherwise to avoid desert.
最终动植物会找到这些上涌的水，在此生长，形成一个微型生态系统，一个生物众多的小世界，也能避开沙漠。
And if conditions remain quote-unquote favorable, oases can exist for thousands of years, virtually unchanged.
如果条件保持在所谓的有利状态，绿洲能够存在上千年几乎不变。
In fact, in 2007, in the Siwa oasis, an early hominid footprint was discovered fossilized in the rocks.
事实上，2007年在锡瓦的岩石中，发现了一个早期原始人的足迹化石。
With estimates for its age ranging as far back as 3 million years, illustraing just how long an oasis can remain intact while completely isolated from the outside world.
据估计，它的年代可追溯到300万年前，这表明，当绿洲与外界完全隔离时，能长时间保持完好无损。
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