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 mortgage insurance specialist MGIC Investment (NYSE: MTG) look anything but subprime today, rising as much as 12.7% on about three times the average trading volume.

So what: The entire mortgage insurance industry is actually booming today with Radian Group (NYSE: RDN) rising 8% and PMI Group (NYSE: PMI) recording a heart-stopping 24% gain. The big driver of this action is PMI Group, which received a somewhat favorable credit review overnight from Standard & Poor's -- the others are simply tagging along.

Now what: As nice as the upticks might seem, all three PMI giants have lost more than 33% of their value in 2011 even after this dramatic change of their luck. An equal-weighted portfolio of all three stocks would have lost 93.5% of your invested dollar over the last five years. However, these stocks could still be fine opportunities for bottom-fishing value hounds: MGIC has inched closer to profitability in recent quarters, and the shaky balance sheet has stopped getting shakier. Just remember to load up on antacids before investing in this sector -- even now.

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Fool contributor Anders Bylund holds no position in any of the companies discussed here. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool is investors writing for investors.