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Credit Suisse's stock is plunging as investors fear a banking crisis.
  • UBS is officially buying Credit Suisse after regulators rushed to restore confidence.
  • Credit Suisse has long been a troubled bank, posting billions in losses and deposit outflows. 
  • Here's a closer look at why regulators are so worried about Credit Suisse.

UBS is officially buying Credit Suisse in an effort to prevent the bank's collapse, the Swiss National Bank announced Sunday.