In downtown Houston there is a 6-mile network of underground tunnels and second story catwalks that allow pedestrians to walk to many popular buildings without experiencing the summer heat. I often made use of these between the classes I taught at the University of Houston, not because I could not tolerate the heat, but because it allowed me to walk indefinitely without having to cross any streets. Many of these tunnels had shops, restaurants, and spaces for art, so there was much to see along the way. Given the expense needed to supply air conditioning to this system I do not think there was anything “environmentally friendly” about it. In London there are plans to convert unused underground train tunnels into spaces for bicycling and walking. To make it more sustainable, the energy for illuminating these tunnels will be produced by “Pavegen” technology: whereby the kinetic energy in people’s footsteps is harnessed to generate electric current.