WEBVTT
Kind: captions
Language: en

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Welcome to Calipatria, a desert town near
California's Salton Sea.

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This is the city with the lowest elevation
in the western hemisphere,

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55 metres below sea level.

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And here, we've also got a visual demonstration
of just what that means:

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because that white spindly thing is a flagpole,

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and the American flag up at the top is flying
above sea level.

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If it wasn't for all the mountains and hills
between here and the Pacific Ocean,

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then we'd be wet,
but that flag would stay dry.

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Maybe.

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We're dry because no rivers
drain into this basin.

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The Salton Sea over there only exists because
some engineers screwed up in 1905

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and diverted a good bit of the Colorado River
in there for a couple of years.

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That water is steadily getting saltier and
more polluted as it slowly evaporates,

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and California's got to deal with that over
the next couple of decades.

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But how do we know we're
55 metres below sea level?

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What is sea level?

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In theory, it is the average height of the
sea around the Earth.

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But there are some problems with that.

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Problem 1: the Earth isn't a sphere.

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I mean, it's vaguely a sphere, don't get too
excited, Flat-Earthers.

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It's a bit squashed.

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The force of its own spin makes it bulge by
a few kilometres around the equator.

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And when you're defining height above sea
level in metres, that's a lot.

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So geographers solved that by defining the
Earth as an oblate spheroid.

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Basically a smooth squashed sphere, just in
fairly precise mathematical terms.

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But then there's problem 2: the Earth isn't
one uniform lump of rock.

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Bits of the mantle are more dense than others,

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so gravity isn't quite the same everywhere
on Earth's surface.

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Which means there's a bit more water sitting
over those bits.

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And by a bit more, I mean anything up to 80
metres more.

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These aren't small amounts,

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that could be the difference between this
city being above and below sea level.

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And to complicate things even more,

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mountains are big enough that their gravity
attracts the ocean, just a little.

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So during the 20th century,

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as scientists and Cold War governments started
to need a global reference system

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for spacecraft, and missiles, which were basically
the same thing back then,

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they spent a long time measuring Earth's gravity.

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The result is the Earth Gravitational Model,
megabytes in size,

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which asks, in purely mathematical terms:

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if this land wasn't here,
but its gravity was,

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and if there was exactly enough water
to fill in the gaps...

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where would all that water sit?

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And how far away would it be from that theoretical
smooth squashed sphere?

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The model was finished in 1996.

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It is available to this day on a web site
that seems unchanged since 1996,

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and it's what your phone and GPS use.

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There are more recent, more precise models,
but they are hundreds of megabytes in size,

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and everyone's agreed that
now we've got a standard,

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let's not change it and confuse everyone.

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Also, we'd also have to update
for sea level rise,

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whether it's best-case centimetres
or worst-case metres.

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So saying we're "55 metres below sea level"
is mostly just symbolic.

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But so are many things in the world,

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and I'd like to think
we still put some trust in them.