MULBERRY — The massive sinkhole that opened up last month at a Mosaic phosphate processing plant near the Hillsborough-Polk county line may be even deeper than previously thought.

The sinkhole — initially reported as being 300 feet deep — may extend much farther down into the ground.

"We believe it's deeper than that," Mosaic executive David Jellerson told reporters on Tuesday. "So the Floridan aquifer — I don't have the exact dimensions, but just for an example, our recovery well goes down approximately 750 feet."

The recovery well is what Mosaic drilled near the hole to try to retrieve the hundreds of millions of gallons of contaminated water that poured into the earth when the sinkhole opened. And the deeper the sinkhole goes, the more serious the threat to the aquifer, which supplies much of Florida's drinking water.

Jellerson, a senior director for environmental and phosphate projects at Mosaic, revealed his concerns on the day that Gov. Rick Scott flew over the sinkhole and toured the site. A company spokeswoman later denied that the 750-foot depth of the recovery well meant the sinkhole itself could go as deep as 750 feet.

However, Mosaic officials also said they don't yet know how deep the hole is. They cited one engineer who predicted it could turn out to be less than 300 feet deep. In an email sent out late Tuesday, a Mosaic spokeswoman said the company believes that later, lesser depth estimate was "based on most current information" and "more precise" than what Jellerson told reporters that morning.

If the hole does go 750 feet deep, it would be one of the deepest ever in Florida, experts said. The world's deepest known sinkhole, Mexico's Cenote Zacaton, is more than 1,000 feet deep.

"It's certainly the deepest I've heard of in sometime," said Robert Brinkmann, a Hofstra University professor who wrote the 2013 book Florida's Sinkholes.

At 300 feet, the sinkhole already spelled trouble for the aquifer. If it were to go as deep as 700 feet, the danger to the drinking water supply would be even greater.

When the 45-foot-wide sinkhole opened up on Aug. 27, it swallowed an estimated 215 million gallons of water from atop one of the processing plant's massive gypsum stacks. It took three weeks for the public to learn about the hole, however.

That alarmed local residents, who feared the contaminated water could taint their wells. So far more than 600 have requested Mosaic to test their drinking wells and provide them with bottled water until it's declared clean.

If the hole sank 750 feet into the earth, Brinkmann said, that would magnify the risk to the aquifer. If the contaminated water were to reach one of the aquifer's underground streams, the professor said it would spread much more quickly and could then become unrecoverable.

"If that sinkhole entered an underground stream," Brinkmann said, "it would be difficult to assess where that water would be at this point."

Mosaic will keep its recovery well pumping water back up from the aquifer as long as it takes to get the contamination out, Jellerson said. However, Mosaic could not say Tuesday how much had been recovered.

"It will operate until it's no longer needed to recover," Jellerson said of the recovery well. "It'll be years."

But that contaminated water poses another problem: It could lead to more sinkholes.

The water is acidic — as acidic as a Coca-Cola, according to Mosaic — and laced with sulfate and sodium. As it sank into the ground, so did an unknown amount of gypsum, a fertilizer byproduct with low levels of radiation.

That acid can eat through the limestone that's beneath the surface of the ground, Brinkmann said, which could create more sinkholes in the vicinity.

The governor visited the phosphate mine as Mosaic officials try to make amends for keeping the sinkhole quiet for three weeks — as did Scott's own Department of Environmental Protection.

Even when Mosaic did acknowledge to government officials that something had happened, the company avoided even using the word "sinkhole." In a report to Polk County officials, a Mosaic consultant said the water drained out of the gypsum stack because of "an anomaly likely connected to the Floridan aquifer system."

Mosaic has apologized for keeping the incident quiet. Scott, during a brief news conference after Tuesday's tour of the site, defended DEP for not speaking up. When the agency finally informed the public earlier this month, DEP officials said they went beyond what they were required to do.

"The DEP followed the existing law," Scott said — but then added that the law made no sense.

The law, passed in 2005, said no one had to tell the neighbors unless the pollution had been detected moving beyond the boundaries of Mosaic's own property.

That's different from what happened with a similar sinkhole that opened up at the same Mosaic site in 1994. Back then, the public was informed within the week.

On Monday, Scott ordered DEP Secretary Jon Steverson to change the rules so that any company or local government that dumps pollution anywhere in the state has to notify the public via the news media within 24 hours. DEP itself, however, would have no obligation to notify the public.

Scott also said the fines for violating that rule would need to be made more stringent to ensure compliance. The governor on Tuesday said he personally came up with the change, not Steverson or anyone at DEP.

"I directed DEP to do it," Scott said. "It's based on my experience in business. When something like this happens you say to yourself afterward, what can we do better?"

In response to a reporter's question about whether Steverson might lose his job over keeping the incident quiet for three weeks, Scott said, "I'm not going to fire Secretary Steverson over anything we know today. We're in the middle of an investigation."

Steverson did not meet with reporters. Neither did federal officials from the Environmental Protection Agency, who Mosaic said were also on site for the briefing.

Scott said his top priority was making sure the pollution that fell into the aquifer does not taint the water used by the plant's neighbors.

"The big thing is to make sure our water is safe to drink," the governor said more than once.

Scott was asked to describe what the sinkhole looked like from the air. He compared it to a hole that opens up in the sand at the beach.

When a wave hits, the governor said, "you can see the water draining down into the sand. It's all messed up."

Times senior new researcher Caryn Baird and staff writer Christopher O'Donnell contributed to this report. Contact Craig Pittman at craig@tampabay.com. Follow @craigtimes.