This city was a lake, and the volcano is still active.
Taipei occupies the floor of a basin that dropped along a fault, filled with water when lava plugged its outlet, and drained some thirty thousand years ago. To the north, fifteen kilometres from downtown, sits a volcanic group classified as active with a magma chamber beneath it. The whole city is explained from there.
01 A city at the bottom of a hole
The fault responsible has a name and a file. It was identified in 1939 by the Japanese geologist Tan Keinosuke; it is a normal fault dipping 50 to 60 degrees with some 35 kilometres of onshore trace, running from Shulin northeast through Luzhou and Guandu. In 2012 the geological survey classified it as a Category II active fault — that is, one that has moved within the last hundred thousand years.
On top of that sunken floor, four Quaternary sedimentary formations piled up. The surface one, the Songshan Formation, is an alternation of silts and sands: lake mud, essentially. It is the material almost the entire city stands on.
02 The lake that was here
Here comes the part that is hard to believe while looking at the city. Around 200,000 years ago, a basaltic lava flow plugged the outlet of the Tamsui estuary. With the drain blocked, the basin filled: the ancient Taipei Lake formed and covered the entire floor for tens of thousands of years.
Around 6,000 years ago, postglacial sea-level rise pushed salt water back in and there was a brackish lake phase. After that, sediment and the river finished the job. The plain Taipei rests on is, literally, the floor of that lake.
And there is a second lake, far more recent and far more disputed. In 1697 the official Yu Yonghe wrote in his travel diary that, after passing the narrows at Guandu, "the water suddenly broadened, like an expansive lake with no visible shore", with some twenty thatched dwellings nearby. It has been proposed that the 1694 earthquake, of estimated magnitude 7.0, dropped the ground enough to let the sea in. Specialists disagree: some calculate 150 square kilometres of water, others thirty, and others hold that Yu was simply describing a bay. It is worth telling for what it is — a written account and an open question, not an established fact.
03 An active volcano fifteen kilometres away
The Tatun volcanic group begins fifteen kilometres north of downtown and gathers more than twenty volcanoes and cones. The highest is Mount Qixing at 1,120 metres, which is also the highest point in the Taipei municipality. It had two eruptive stages, between 2.8 and 2.5 million years ago and between 800,000 and 200,000, and its most recent activity is placed some 5,000 to 6,000 years ago.
That last figure changes everything. Under the international criterion that treats as active any volcano that has erupted within the last ten thousand years, Tatun is one. The evidence has accumulated: a magma chamber of some seventy square kilometres at around twenty kilometres depth, inferred from the delay of seismic waves; helium-4 ratios between 4 and 7, characteristic of the mantle, against a crustal value of 0.1; vertical ground deformation of about nine millimetres between 2008 and 2009; and fumaroles and hot springs that have never stopped working.
In October 2011 the Tatun Volcano Observatory was created inside Yangmingshan National Park, precisely to monitor it. The park, designated in September 1985 over 113.38 square kilometres, had already been a national park in 1937, under Japanese administration, under a different name.
04 The mineral that exists in only two places
The heat of that system reaches the surface at Beitou, a northern district you can get to by metro. Thermal Valley reaches 90 degrees and has a pH of between 1.4 and 1.6: it is an acid sulphate spring, aggressive enough that its composition includes gypsum, alunite, jarosite, realgar, sulphur and radium.
From that water precipitates a mineral called hokutolite, a radioactive lead-bearing variety of baryte. It was discovered in 1907 by Okamoto Yohachiro and formally named in 1912 at a mineralogical congress in Saint Petersburg, using the Japanese reading of Beitou. It is the only mineral in the world named after a place in Taiwan.
And the genuinely rare part: it has been confirmed at only two points on the planet, the Beitou spring and the Tamagawa spring in Japan's Akita prefecture. Japan declared it a special natural monument in 1952, Taiwan protected it as a natural cultural landscape in 2000 — the first mineral to receive that status — and in 2013 the city created a dedicated reserve for it.
05 Building on top of this
A basin of lake mud, an active fault underneath and an average of three direct typhoons a year are not a good starting point for building tall. The most visible answer to that problem is Taipei 101: 508 metres, 101 floors, opened on 31 December 2004 and the tallest building in the world between March 2004 and March 2010.
The interesting part is not the height but what it carries inside. Between floors 92 and 88 hangs a steel sphere of 660 tonnes and 5.5 metres in diameter, made of 41 circular plates 125 millimetres thick each. It is a tuned mass damper: it swings against the building and cuts up to 40% of its movement. It cost 132 million Taiwan dollars and is calculated for winds of 216 km/h and for an earthquake with a 2,500-year return period.
On 8 August 2015, Typhoon Soudelor swung that sphere a full metre, the largest displacement ever recorded. And the sphere is open to visitors: it is seen from the observatory, between floors 88 and 92. Few buildings show the public the piece that keeps them standing.
Some 2.43 million people live in the city across 271.8 square kilometres, and the figure has been falling for years. Around it, New Taipei encloses it completely with more than four million, and the metropolitan area with Keelung exceeds seven million: roughly a third of Taiwan's population, all resting on the floor of the same dry lake.
