How about a balloon that opens within a subway tunnel to block the water flow enough to allow any leakage to be pumped out?
|An inflatable device that could save tunnels from flooding. By HENRY FOUNTAIN|
The Netherlands has been struggling with this problem for its entire existence. Here are the flood gates that close periodically to protect Rotterdam from the ravages of the North Sea:
Tineke Dijkstra/Hollandse Hoogte