A HUNDRED years ago, summer day-trippers were lured on to London Underground’s Bakerloo line with the promise that, at a refreshing 15 degrees centigrade, its tunnels were the coolest place in the capital.
   Times have changed. Decades of operation have heated the very soil through which the tunnels run and led to the sweaty climate familiar to today’s passengers. On August 24th Tube officials published a heat map of the network. The Bakerloo line is baking, with temperatures in its central London tunnels exceeding 32 degrees. The Central Line is above 32 degrees at every station between luxurious Holland Park in the west and Bethnal Green in the East End [90 Fahrenheit].
   Relief is in sight for some: from 2010 air-conditioned trains will begin to appear on the four sub-surface lines. But the deeper lines are trickier: space is so tight that there is nowhere for waste heat to go, which makes traditional air-conditioning impossible. More creative ideas are being explored. One being tested at Victoria station pumps river water through heat exchangers. It works well, but is only suited to a handful of other places. Other ideas include drilling into the aquifer beneath the city and pumping cool water along the tunnels (a method already used to cool the Royal Festival Hall), or installing chillers in the trains that would freeze blocks of ice while the trains were above ground—which would then keep them frosty on the underground sections.