A billionaire investor from Colorado, an entrepreneur from Texas and a pair of prospectors looking to strike it rich in Nevada. They are among the top holders of idle oil and gas leases on U.S. federal lands, according to a Reuters review of government data.

Such companies and individuals that have stockpiled vast amounts of public drilling acreage in recent years are now on the hot seat as the administration of President Joe Biden on March 25 launches a formal review of the leasing program to weigh taxpayer value against environmental costs.

Biden froze new drilling lease auctions as one of his first moves in the White House to pave the way for the review, triggering a backlash from the oil and gas industry, which warned it would cost jobs and hurt the economy.

But a chief complaint of Biden’s administration is that half the 26 million acres currently under lease are not producing, generating little for public coffers, while the rest is contributing to climate change.

According to the Reuters review of Bureau of Land Management leasing data, the top five holders of non-producing leases control about 1.5 million acres of federal land, an area slightly smaller than the state of Delaware.

The biggest among them is Kirkwood Oil & Gas, a 50-year-old Rocky Mountain exploration company that holds nearly half a million non-producing acres, according to the data.

That is part of the business model, according to Steve Degenfelder, the company’s land manager, likening exploration to “a high-stakes game of Battleship,” the strategic guessing game.

“The last thing you want to do is identify a good drilling target, and then you don’t own the lease on it,” he said. “So the leasing starts way, way before anybody even thinks of drilling a well.”

He said Kirkwood’s business revolves around exploring for oil prospects and then only selling them to drillers once there are enough contiguous leases over the same deposit to package together—something that can take years.

The second biggest holder of idle leases is an enterprise run by business partners Stephen Smith and Larry Moyer, residents of Colorado, who are betting on a future drilling boom in the unlikely state of Nevada. They have 300,000 non-producing acres, mostly in Nevada.

“It’s got very complex geology,” Smith said.

The third is Anschutz Exploration Co., part of the business empire of Denver-based billionaire Philip Anschutz, which has around 275,000 non-producing acres.

Joe DeDominic, president of Anschutz Exploration, said it is hard for an exploration company to identify and test petroleum deposits when federal, state and private land auctions in the West tend to sell only small parcels at a time.

“It can take upwards of a decade or more to put all those rights together to justify spending the money and resources to test your theory,” he said.

Most of Anschutz’s acreage is in the Powder River Basin in Wyoming, a state with more than a third of the 13.9 million acres of non-producing federal land, according to BLM.

The Department of Interior declined to comment for this story.

The federal leasing program has not reformed royalty rates or minimum bids for decades, which critics say has depressed taxpayer returns and made it easy for speculators to amass large amounts of acreage on the cheap.

Former President Donald Trump sought to maximize oil and gas production on federal lands by leasing 26 million acres over four years at a time of depressed industry demand.

Other major holders of non-producing acreage include Liberty Petroleum, a New York-based firm with 270,000 acres, and Texas entrepreneur Avinash Ahuja, who holds more than 210,000 non-producing acres. Neither could be reached for comment.