Looking back over 2020 with only three days remaining, what a stressful year it’s been. Stock markets rose steadily until February, when fears of the Covid-19 pandemic smashed share prices. By 23 March, the FTSE 100 index had collapsed below 5,000, down 2,550 points — around a third (33.8%) — in three months. In the US, the S&P 500 crashed almost 1,000 points (30.7%), also bottoming out on 23 March. However, the US market has since rebounded to all-time highs. To me, this suggests that we are in the fourth market bubble of my lifetime.Bubbles cause crashesHaving been an investor during the 1987, 2000-03 and 2007-09 stock-market crashes, I’ve learnt painful lessons from these three meltdowns. As a result, I don’t buy assets at sky-high prices, because I know this will reduce my future returns. Likewise, I steer clear of frothy markets during their periodic bubbles. How do I identify a bubble? It’s hard to say, but I know one when I see one! Here are three market bubbles that I see as having inflated to dangerous levels in 2020.5G is here – and shares of this ‘sleeping giant’ could be a great way for you to potentially profit!According to one leading industry firm, the 5G boom could create a global industry worth US$12.3 TRILLION out of thin air…And if you click here we’ll show you something that could be key to unlocking 5G’s full potential…My three bubble marketsThe price of Bitcoin has soared this year, rising by $20,000 from around $7,200 to all-time highs over $28,200 yesterday. Why is Bitcoin up 280% in 2020? There is no way of knowing and no fundamentals to review. All that has driven the Bitcoin price higher since March has been buying pressure and momentum. For me, that’s good enough to declare Bitcoin as my first market bubble. In 2021, I expect Bitcoin to perform as it did in 2018 — by crashing hard when financial gravity finally overcomes speculative excess.I call US tech stocks as my second market bubble. The NASDAQ Composite tech index has soared by three-sevenths (42.7%) this year, setting record highs in late 2020. For me, the NASDAQ in 2020 has echoes of its performance in 1999/2000. Twenty years ago, the tech index peaked above 5,000 points on 14 March 2000 (coincidentally, my 32nd birthday). By winter 2002, it had lost four-fifths (80%) of its peak value. Today, the tech index is dominated by the ‘Big Five’ mega-tech firms, all huge and profitable. But the NASDAQ trades on a price-to-earnings ratio of 39 and an earnings yield below 2.6%. For me, this indicates that US tech stocks are looking bubbly.My third bubble is in market listings — IPOs (initial public offerings) and SPACs (special purpose acquisition vehicles). Again, the first-day performances of over-hyped IPO stocks and SPAC shares have scared me this year. Indeed, the opening ‘pops’ (price spikes) in companies such as cloud-software provider Snowflake and delivery firm DoorDash are only beaten by the 2000 vintage of IPOs. And we all know what happened to those bubble stocks over the next couple of years, don’t we?The FTSE 100 is no bubbleWhile I see multiple bubbles in the US, partly driven by the ‘Robinhood herd’ of younger investors, I see great value in UK-listed stocks. The FTSE 100 currently hovers around 6,500 points. The Footsie first passed that level in 1999, and is also over 1,400 points — more than a sixth (17.7%) — below the record high reached in May 2018. What’s more, the price-to-earnings ratios, earnings yields and dividend yields of many individual FTSE 100 businesses are extremely attractive today. That why, in 2021, I will move some of my family wealth out of over-priced US stocks and into cheaper FTSE 100 value shares. I believe this will surely produce better returns over the next 10 years, helping me to retire rich! Cliff D’Arcy | Monday, 28th December, 2020 For me, 2020 was a huge market bubble. But will 2021 be a crash year? Enter Your Email Address Simply click below to discover how you can take advantage of this. Adventurous investors like you won’t want to miss out on what could be a truly astonishing opportunity…You see, over the past three years, this AIM-listed company has been quietly powering ahead… rewarding its shareholders with generous share price growth thanks to a carefully orchestrated ‘buy and build’ strategy.And with a first-class management team at the helm, a proven, well-executed business model, plus market-leading positions in high-margin, niche products… our analysts believe there’s still plenty more potential growth in the pipeline.Here’s your chance to discover exactly what has got our Motley Fool UK investment team all hot-under-the-collar about this tiny £350+ million enterprise… inside a specially prepared free investment report.But here’s the really exciting part… right now, we believe many UK investors have quite simply never heard of this company before! Image source: Getty Images. Our 6 ‘Best Buys Now’ Shares The high-calibre small-cap stock flying under the City’s radar Renowned stock-picker Mark Rogers and his analyst team at The Motley Fool UK have named 6 shares that they believe UK investors should consider buying NOW.So if you’re looking for more stock ideas to try and best position your portfolio today, then it might be a good day for you. Because we’re offering a full 33% off your first year of membership to our flagship share-tipping service, backed by our ‘no quibbles’ 30-day subscription fee refund guarantee. Cliffdarcy has no position in any of the shares mentioned. The Motley Fool UK owns shares of and has recommended Snowflake Inc. Views expressed on the companies mentioned in this article are those of the writer and therefore may differ from the official recommendations we make in our subscription services such as Share Advisor, Hidden Winners and Pro. Here at The Motley Fool we believe that considering a diverse range of insights makes us better investors. See all posts by Cliff D’Arcy I would like to receive emails from you about product information and offers from The Fool and its business partners. Each of these emails will provide a link to unsubscribe from future emails. More information about how The Fool collects, stores, and handles personal data is available in its Privacy Statement. Click here to claim your copy of this special investment report — and we’ll tell you the name of this Top Small-Cap Stock… free of charge!