Imagine the dilemma of having so much cash in your bank account that you didn't know what to do with it. This pipe dream for the average American is now reality for the country's biggest corporations. The industrial companies that make up the Standard & Poor's 500 index...have a staggering $643 billion in cash and equivalents. We're in a time that is out of whack with all historical numbers," said Howard Silverblatt, equity market analyst at Standard & Poor's. "People are demanding why corporations need so much cash, what are they going to do with it?" Companies began propping up their reserves through 16 straight quarters of double-digit profit growth. Leading the pack with the most cash is Exxon Mobil Corp., which has about $36.55 billion on its balance sheet. That amount is nearly equal to its 2005 profit of $36.13 billion, the highest ever for a U.S. company. Some results of the cash riches: An unprecedented $500 billion of stock buybacks. Last year, ExxonMobil spent $18.2 billion buying its shares. One of the biggest avenues in which companies have spent this excess money has been through mergers and acquisitions. Some 75.4 percent of all deals under $1 billion so far this year were done purely with cash.