Look at this church. Since it was first built, it has started to sink into the
ground below. And it’s far from the only building like this in the city.
When that monument behind me was first built about 100 years ago,
the base only had 9 steps. However since then, the streets around it have sunk
so much that they’ve had to add another 13. This is Mexico City, the largest city in
North America, and the fifth biggest city in the world. It is bigger than New York,
rainier than Portland, and higher than Denver (well, maybe not in both ways). More than 20
million people call this megacity’s metro area home . But Mexico City has one major problem:
water. The city conquistadors once called “the Venice of the New World” is sinking.
And the city that gets more rain than London or Vancouver is in danger of running out of water.
So what is happening in Mexico City? What are people doing to save the city that 1 out of every
6 Mexicans calls home? And how did it beat New York to become the biggest city on the continent?
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Mexico City has sunk by over nine meters in some places over the last 100 years alone.
Buildings across the megacity are bent and tilted by not insignificant margins.
This is more obvious when looking at older buildings, like churches and monuments,
but even newer apartment buildings suffer from these effects,
that are often nauseating to live in. Take a look at this map. This is a map of buildings that were
damaged during an earthquake in 2015. But this cluster of buildings over here is no coincidence.
Trace out the map and we see what this area was before an entire city was built
on top of it. 500 years ago, this area was not the sprawling megacity it is today,
in fact most of it wasn’t even dry land. This is Lake Texcoco, or rather *was* Lake Texcoco.
According to legend, when the Aztec (or Mexica) people left their homeland of Aztlan 700 years
ago, the sun god Huitzilopochtli directed them toward a swampy island in the middle of the lake
through the sign of an eagle eating a snake while perched atop a cactus, which is what that thing on
Mexico’s flag represents. Atop this unassuming island would form Tenochtitlan, capital of the
new Aztec Empire. The city would gradually expand through the use of small, rectangular
artificial islands known as chinampas. In only two centuries, Tenochtitlan had grown so large
it could have supported upwards of 200,000 people. Unfortunately, the Aztec Empire wasn’t exactly the
most liked kid on the block when Hernán Cortés and co. showed up in 1521. The city would be
besieged by the Spanish forces and their native allies, who would almost completely destroy
the city and bury it under a new administrative center for a different empire. But Cortés was the
only one amongst the conquistadors who wanted to build their new city atop the old Aztec capital,
most of the others wanted to build somewhere that, you know, actually made sense.
But this decision was Cortés’ to make alone, and he ordered the new Spanish city be built on the
ruins of Tenochtitlan, to erase it from history. From the newly built main square, the Zócalo,
the new Mexico City extended outward in a typical Spanish colonial traza,
or city plan. As the centuries passed however, the city inched closer and closer to the lake. Since
it was first founded, the city has been regularly plagued by flooding, an issue that still plagues
the city to this day. So the entrances to the metro stations are raised slightly,
like they do in Southeast Asia. Lake Texcoco, and now Mexico City, sits within a vast plateau,
two kilometers above sea level, and surrounded by mountains reaching over 5km into the air.
What this means is that Mexico City is structured almost like a bowl, with no natural way to drain
excess water in the lake. In fact, as early as the 17th century, leaders and residents of the
city considered moving the capital somewhere like nearby Puebla, whose position had the distinction
of… actually making geographic sense. But they didn’t move the capital, and
insead embarked on a centuries-long project, the desagüe (literally meaning drain), conscripting
indigenous workers to dig tunnels and trenches to drain water from the lake. At first this project
more or less… failed miserably, and it took another 200 years for the project to start again,
with some actual traction. Over the 19th and 20th centuries, more channels were built as the lake
was drained even further into surrounding rivers. This didn’t just give the city more
space to build on, but also kept mosquitoes and sewage– and therefore disease– out of the area.
But then, something else happened to the city: it received even more humans. In 1950, the city
had a population of only 3.3 million, but then people from all across Mexico started to move in,
with the prospect of new jobs and new opportunities in the industrial sector.
Now, 70 years later, the population sits close to 22 million. And two things people generally need
are water and shelter. But ironically for a city that gets more than a meter of rainfall a year,
many parts of the city don’t have a stable supply of drinking water. The city gets most of its water
through underground aquifers. Normally these aquifers would be replenished by rain sinking
through the soil until it reaches the aquifers. However rain can’t penetrate concrete like it
can bare soil, and as rainwater is pumped away to prevent the city from flooding,
the aquifers can’t be replenished, and so the soft soil above them begins to sink.
But this groundwater pumping doesn’t explain *all* of the subsidence that has been plaguing the city.
The simple fact that it was built on a dried lakebed at all is causing the city to sink.
The city is built atop super soft sand and soil, which itself is more water than solid in some
places, and the buildings often have shallow foundations, allowing them to sink possibly
by up to 30 meters over the next 150 years. Now let that sink in for a moment… oh, right, sorry.
Something that helps this even less is earthquakes. If we zoom out,
we can see that Mexico City sits within a valley amidst the Trans-Mexican Volcanic Belt,
a string of active volcanoes stretching from coast to coast across Central Mexico. Perhaps
the biggest example of this was the magnitude 8.1 earthquake that struck the city in 1985,
killing more than ten thousand people and causing hundreds of buildings to collapse as the sand
below gave way. This happened through a process you’ve probably heard about on some of those
nerd channels called liquefaction, where sand starts to behave like a liquid when agitated–
say by an earthquake– causing the buildings on top to sink like they were built on water.
But if Mexico City was built on a lakebed in an area that gets a lot of rain,
why are so many of its residents struggling to get water? Poor wastewater treatment, old pipes that
leak 1,000 liters of water per second, and the simple fact that the aquifers are being depleted
mean that many in the outskirts of the megacity have to rely on water being delivered by trucks.
Nonprofits like Isla Urbana seek to help give Mexico City’s residents the ability to store
rainwater, but… well let’s see, is climate change affecting things? Of course climate
change is affecting things. The city has been facing increasingly severe droughts,
with researchers predicting the city will receive 10-17% less rainfall by 2050, which
will severely strain the water supply for tens of millions of people, and make wildfires worse.
Ultimately– short of putting floaties on the city’s buildings– once any
piece of land within the city sinks, it cannot be un-sunken.
I’m not gonna go out and say it’s over for Mexico City and that the Mexican government
should finally make the move to Puebla and that you shouldn’t go to this city (you
100% should!). The city isn’t gonna need to be evacuated or get swallowed into the Earth,
but it will need help going forward. Mexico City is not the only city facing a sinking problem–
Jakarta is facing a very similar issue– but Mexico City’s geographic (and geological) problems are
quite unique amongst the major cities of the world. So uh… I guess it has that going for it.
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