Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Cliffs Natural Resources (NYSE:CLF) plunged an eye-popping 20% today after the company announced that it was moving to shut down the Bloom Lake mine in eastern Canada.

So what: The 2011 $5 billion acquisition of Consolidated Thompson that brought the Bloom Lake mine into the fold has been a headache for Cliffs. Now, with iron ore prices at their lowest since the bottom of the recession, the mine is losing the company money hand over fist. Cliffs had two options: 1) Find equity partners to help pay for the second phase of the mine's development, or 2) shutter the mine. Considering the amount of new iron ore supply coming online today, that first option was looking less and less attractive. Cliffs said in a press release that closing the mine will cost between $650 million and $700 million over the next five years

Now what: Why this announcement sent so many people headed for the exit is anyone's guess. Cliffs CEO Lourenco Goncalves gave the same message during the company's earnings call just a couple of weeks ago. Many people may be scared of the $700 million charge Cliffs will pay to close the mine, but the company has already paid $160 million this year trying to operate its Canadian mines.

Over the long term, this will be the right move for the company. There is simply too much iron ore coming online from the Australian mining giants, and marginal-cost tons of iron ore like those from Bloom Lake aren't going to cut it unless we see a sudden monumental shift in demand for iron ore and steel. Cliffs might face a few years of pain as it pays to close the mine, but once that happens, the company should emerge a leaner, meaner producer for investors.