Different cultures have different ways of coping with extreme heat. In Houston, Texas, when the mercury rises, office workers head underground rather then go outdoors. Their refuge is an elaborate network of tunnels, which has expanded over the years without any central planning. Malcolm Brown paid a visit.

Looking at Houston's high-rise landscape, you might imagine that the streets would be packed at lunchtime. But they're not.

That is because many office workers are down here, in a network of pedestrian tunnels around 11 kilometers long.

Sandra Lord, also known as 'the tunnel lady,' provides tours of this labyrinth, which can be hard to navigate. "People who walk around it, if they see a tourist or someone who looks lost, they'll walk up to them and say, 'Can I help you?' They know that to be polite you walk on the right, so that people can come the other way on the right. There's a passing lane."

The popularity of these air-conditioned tunnels stems from the heat outside. Top temperatures in summer average around 33 degrees centigrade. High humidity makes it feel even hotter.

Down here in the cool there are over 100 places to eat, as well as a host of service providers; including banks, doctors, dentists, and florists. Some people even walk the corridors for exercise.

Sandra Lord also runs above-ground tours, but she became fascinated by the world beneath. "It's the oldest and largest, tunnel-skywalk, mostly private, completely pedestrian system in the world! (Laughs)"

It is mostly private, because each section belongs to a building above. The patchwork ownership accounts for the uncoordinated growth, as developers and property owners add to the system.

Susan Speck, from Tarantino Properties, shows off an area that will soon be a new food court. Getting the word out is easy. "Actually, the tunnel is a very effective way to advertise because there's so much foot traffic. People walk by here every day at lunchtime and we can put up a sign like this and it's a very effective way to get out the word. People see that sign, they call us, they inquire about the space."

In a city known for its reliance on air-conditioning, the subterranean network is expected to grow. There are already 77 buildings connected.