Up to 24 inmates are crowded into a single cell, living behind metal bars on steel bunks, sharing a single shower and two toilets.
Little of the bright Miami sun filters through the grilles on the windows. Visits to the yard happen twice a week for an hour. The rest of the time, inmates are holed up round the clock, eating, sleeping, and going slightly crazy.
But what is most shocking is the behaviour of the inmates themselves. For reasons that remain to some extent opaque - perhaps because of the bleak conditions they live in or because of insufficient supervision by officers, maybe because they lack other outlets for their energies, or because of their involvement with gangs on the outside, or maybe from a warped jailhouse tradition - the incarcerated here have created a brutal gladiatorial code of fighting.
They fight for respect, for food and snacks, or simply to pass the time.
The whole thing seemed so futile. Damaged boys arrive at the jail to be brutalised and criminalised by years of imprisonment and humiliation. With Theroux's personal style drawing out the dark stories of individual members the viewer is confronted with the ugliness of the system, but there is little chance of a wider view: we do not get to see if this system really works or not, what impact it might have on society.
As it happens, the wider view of Florida's crime environment is surprisingly positive. Since 1993 the state of Florida has grown by 5 million people, yet the total number of violent crimes has fallen from 161,789 to 101,906.
That is, as a percentage of population, the number of violent crimes fell by about half between 1993 and 2010. Including non-violent crimes, the rate of crimes per 100,000 people peaked in 1989 at 8,755.9, down more than a half to 4,104.7 by 2010.
Florida's jail system may not be a significant factor in this dramatic improvement in crime rates. But it shows how different the message from Theroux's programme might have been if they had been included. I can imagine another documentary-maker travelling to Miami to celebrate the success of their mega jails and see how it can be repeated elsewhere.