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 Cloud Peak Energy (NYSE:CLD) fell just a hair short of 10% today as prices for Powder River Basin coal have been falling in recent weeks.

So What: Today's 10% drop was just adding insult to injury to Cloud Peak's share price recently. Over the past year shares of Cloud Peak have lost more than two-thirds of their value as coal prices have taken a sharp turn for the worse.

CLD Chart

CLD data by YCharts

Today's pain was especially poignant because the NYMEX spot price for Powder River Basin coal -- the only region in which Cloud Peak operates -- fell below $10 per ton as natural gas prices are also low and taking power generation business away from coal. Also all futures contracts for that particular type of coal are below $10 for the rest of 2015. In its most recent earnings presentation, Cloud Peak's average cost per ton sold was $10.02, which means that the company's already razor-thin gross margins could get even thinner and possibly go into the red unless even more significant cost cutting is done.

Now What: There are just so many things working against coal companies right now -- cheap natural gas stealing power generation market share, tighter emission regulations on power-generating facilities, and China's appetite for coal getting smaller -- it's really hard to see what will happen in the future. Investors who look at this space and say there are a few puffs left on the cigar may be right eventually, but it may take a long time for that to happen.