For the past year, a London Underground team has attempted to come up with some engineering solutions to the problem.
In the deepest tunnels, temperatures regularly reach 30C (86F) in summer.
The trial, which starts this summer, aims to make it cooler for passengers on platforms - but new air-cooled trains are still some years off.
Victoria Tube station, like many on the network, is so deep it is effectively under water and pumps out 35 litres (eight gallons) a second, to stop it coming through the walls.
The new system would see the water being pushed through a network of pipes into heat exchange units on the platforms, which will suck in warm air and pump out cooler air.
If the year-long trial is successful, it could be rolled out to another 30 deep-level Tube stations.
New trains for the Circle, District, Hammersmith and City and Metropolitan lines will have an air cooling system, but are not due to start running until 2009.
As well as cooling trains, LU has to find a way to get the heat out of the underground network.
On the buses too, new measures are being tried out.
New buses - about 20% of the fleet - are built with white roofs, extra windows upstairs and air ventilation systems.
LU has long struggled with how to reduce the heat in narrow Tube tunnels - some built more than a century ago.
It has held several competitions but is still looking for a long-term solution and said it is one of its biggest challenges.
The project team looked at 200 stations and ventilation plant rooms, to find out where the problem was worst.
LU's managing director Tim O'Toole said: "There is no one overall solution to heat on the Tube but significant progress is now being made.
"Some passengers may notice a small difference this summer but we know that there is a lot more to do and it will take many years to cool the Tube."