Rain -- yes, rain -- is causing fires in Metro tunnels, according to new Metro board documents released Monday.

The number of fires reported in the Metro system have doubled in the last few years. Here are the three things you need to know:

1. Water and Metro don't mix. It's powered by the electrified third rail. Water keeps leaking into tunnels, pooling up, and causing sparks. They can be really big and cause a fire.

2. There were 32 fires in the last three months. That, according to just-released documents. Compare that to only 15 during the same period three years ago. That's more than double. Why the increase? You guessed it. Metro says it rained more. There was 9 inches of rainfall in July alone. Metro says the average July rainfall is about 3 inches.

3. Metro is hardly water-tight. To really understand this problem you have to go back in time. The red line is Metro’s oldest line. It was made with older tunneling techniques. Leaks in Maryland can cause fires down here near Dupont.

Metro has a new program to fix this. But that means shutdowns. This weekend, there will be no trains between Medical Center and Grosvenor-Strathmore.