Initial public offerings (IPOs) are the process companies use to tap the public stock markets for capital. They usually involve early-stage businesses that are looking for fresh fuel for growth; many are small opportunities, but a few have mammoth potential.

IPOs aren't guaranteed tickets to riches, as implosions such as Pets.com and Webvan proved. But they can produce explosive returns, and in a hurry. Through the end of September, 38 offerings had produced returns of 50% or more. One thing 2018 has lacked, however, is the mega-deal - an offering such as Facebook (FB) or Alibaba (BABA) that sets the world on fire. That should change in 2019.

Dara Khosrowshahi, CEO of ride-hailing service Uber, says the company expects to execute an IPO next year. "Uber is a widely known brand that connects with millions of consumers and they have also received large amounts of capital from investors over the years," says Scott Coyle, CEO of ClickIPO, an online platform that allows retail investors to purchase IPOs. No wonder then, that The Wall Street Journal reported that proposals from Goldman Sachs and Morgan Stanley valued the company at more than $120 billion. Given that IPOs often raise about 10% to 15% of their projected worth, that could be a $12 billion-$18 billion raise.

Where would that rank among America's biggest IPOs? Today we'll look back on the 25 biggest deals in U.S. history, as measured by money raised.

25. Citizens Financial Group

Courtesy Mike Mozart via Flickr

Royal Bank of Scotland (RBS) paid $440 million for Citizens Financial Group (CFG, $34.92) in 1998 in a bid to expand into the U.S. market. But the global financial crisis of 2007-08 disrupted the company's ambitions. The British government, which owned roughly 80% of RBS as the result of a $42 billion bailout, pushed for a Citizens spinoff.

At the time, the bank was the 13th largest in the U.S. at $130 billion in assets, employing more than 18,000 employees across roughly 1,230 branches in 11 states, primarily in the New England, Mid-Atlantic and Midwest regions.

The IPO was a difficult sell. Interest rates were at historically low levels that made it difficult for banks to generate robust profits, and the U.S. economy's rebound still was viewed as sluggish. The initial price range for the offering was $23 to $25 per share, and Citizens only fetched $21.50. Shares did perk up a bit on the first day of trading, but only by about 7%.

The U.S. economy (and interest rates) eventually perked up, however, bolstering bank stocks. Citizens' stock has grown by 62% from its IPO price, versus about 43% gains for the Standard & Poor's 500-stock index.

24. China Life Insurance

Mao Zedong's government set up the People's Insurance Company of China (PICC) within a few, $10.75).