I got a little too enthusiastic about this prompt and ended up plotting a three-part story that might end up at novella length.
However, the different parts are fairly self-contained, so here's a slightly reworked version of part of the second act, which works as its own action-packed short story. Enjoy!Warrenton
was the first vertical city on Sophra, a planet that had a clean
oxygen-nitrogen atmosphere, but one too thin at the surface for
humans to breathe unassisted. The Vertical City Project was designed
to take advantage of the cool, stable crust of the planet and create
a haven for humanity deep below the surface. Ten kilometers below the
surface, the air was thin but sufficient. Fifteen down, oxygen
partial pressures were comparable to Earth at sea level. At twenty,
the air was thicker and made physically demanding jobs, like mining,
easier and more efficient.
The
massive excavation project had just crossed the twenty kilometer mark
a few weeks ago. The main shaft was about half a kilometer wide and
ample ventilation and modern lighting technology made the deep
underground just as healthy as a day in the sun on Earth.
However,
today there was a problem. One of the three main ventilators failed,
and the other two shut down only a few minutes later. The elevators
stopped working, and the network of radio frequency repeaters that
made communication possible through and around the caverns went down.
The city managers scrambled to get things back online. Without
ventilation, the air down at the bottom of the city would be
breathable for around three hours. Citizens living deep underground
all maintained twelve hour supplies of breathable air, but the clock
was ticking.