Most investors can rattle off some of the best growth stocks of the last decade or two. The tech sector has delivered some of the best growth stocks of all time in Apple, Amazon, and Netflix. A handful of biotech companies typically grace the list, too.

But wouldn't it be great to know about the next big growth stock before it becomes a well-known opportunity? We recently asked three contributors at The Motley Fool for their best under-the-radar growth stocks. Here's why they chose Tellurian (NASDAQ:TELL), iRobot (NASDAQ:IRBT), and Fastenal (NASDAQ:FAST).

Fighter jets flying in formation with contrails behind them.

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Supercooled natural gas for the win

Maxx Chatsko (Tellurian): The fracking revolution has turned the United States into the most important energy producer and exporter on the planet virtually overnight. The country is expected to become a net energy exporter in the early 2020s thanks to booming exports of refined petroleum products, crude oil, and natural gas. That includes an incredible rise in the nation's liquefied natural gas (LNG) infrastructure, which will see U.S. export capacity more than double from 2018 to 2019 -- enough to make it the third-largest LNG exporter in the world behind Australia and Qatar.

The awesome growth opportunity represented by LNG extends to individual investors, too. Cheniere Energy will remain the dominant player, with two export terminals in its fleet, but a $17 billion market cap might leave limited room for growth. That's why individual investors should look to the next wave of export terminals, or those expected to come online in the mid-2020s, for opportunities. Tellurian is currently leading the pack.

A bird's-eye view of a tanker getting filled with LNG.

Image source: Getty Images.

The $2.5 billion company has proposed the Driftwood export facility, which would have a nameplate capacity of 4 billion cubic feet per day (Bcf/d). That would make it one of the largest two or three terminals in the country, right behind the 4.2 Bcf/d fully ramped-up capacity expected at Cheniere Energy's Sabine Pass project. It's not expected to begin operations until 2023, but that doesn't mean investors should wait until then to start a position.

Tellurian could be rewarded for each milestone reached between now and then. And with its equity stake in Driftwood expected to deliver upward of $2 billion in cash flow per year once the facility is up and running, successfully executing on development would certainly earn the company a higher market cap.

Case in point: The business recently received a final environmental impact statement for Driftwood from federal regulators. That sets the stage for construction to begin in the first half of 2019. Investors can then watch for progress updates, the signing of new offtake agreements, and the building of a pipeline (and potential business deals involving the company's nearby natural gas reserves). Admittedly, this might be a boring investment for years, but at a market cap of just $2.5 billion, Tellurian represents an intriguing growth opportunity in American LNG.

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Bet on the bots

Chris Neiger (iRobot): Investors in iRobot, the company famous for its autonomous vacuuming Roomba robot, have been cleaning up lately. The company's share price is up over 100% over the past year, with a huge jump coming last month when iRobot reported strong fourth-quarter results.

Sales increased in the quarter by nearly 18%, revenue for the full year reached $1.09 billion, and earnings per share of $3.07 outpaced management's own guidance.

There have been some fears that iRobot won't be able to perform as well if the U.S. and China launch into a full-blown trade war, but the company tamped down those worries on the fourth-quarter earnings call by saying that it's pursuing a manufacturing partner outside of China. That takes away the threat of new tariffs for at least a few of the company's Roomba devices and should help the company stay on its current growth trajectory.

Management expects 2019 to be even better than 2018, with a handful of new devices launching this year and anticipated sales in the range of $1.28 billion to $1.31 billion, which would be a year-over-year increase of about 18.5% at the midpoint. iRobot has worked hard to earn an early lead in the robotics market, and as more of its consumer devices start connecting to smart-home hubs and riding the smart-home trend, iRobot appears poised for even more growth.

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A rare breed in a cyclical industry

Neha Chamaria (Fastenal): You wouldn't generally consider an industrial tools manufacturer a growth stock given the cyclicality of the industry. Fastenal, however, seems to be an exception, as I recently found out after the company's latest operational numbers caught my attention and encouraged me to dig deeper.

Fastenal has been in business for 50 years, manufacturing machinery hardware and supplies such as fasteners (think nuts and bolts), tools, and accessories essential to keep machines and facilities up and running. In 2018, Fastenal's sales and operating income grew 13% each, while its net earnings jumped nearly 30%. With those numbers, Fastenal proved 2018 was just another good year for the company. Check the pace at which the company's key operating metrics have grown over the past decade.

FAST Revenue (Annual) Chart

FAST Revenue (Annual) data by YCharts.

Even more compelling, Fastenal has consistently generated double-digit -- 15% or more -- returns on equity and invested capital over the years. Those are some of the key characteristics I'd look for in a growth stock, and Fastenal passes the test.

The question is: What is Fastenal doing differently than peers to be able to deliver strong numbers consistently? More than the products it makes, it's all about how it sells those products. So instead of relying entirely on traditional retail stores for sales, Fastenal devised offbeat sales strategies to serve industrial customers from their own facilities, such as vending machines, bin stock, and on-site locations. As of the end of 2018, Fastenal had more than 96,000 vending machines installed and expects to add another 23,000 to 25,000 machines in 2019.

Fastenal shares have quadrupled in the past decade and offer a nice 2.7% dividend yield. While a growth company doesn't usually pay a dividend, Fastenal is a rare mature company that offers strong returns potential to growth and income investors alike.