00:43 Massive Sinkhole Opens Up at Louisville Zoo in Kentucky A massive sinkhole opened up Wednesday at the Louisville Zoo in Kentucky, prompting the zoo to close. Officials say all the animals and people are safe, and an investigation is ongoing to determine what had caused the sinkhole.

At a Glance The sinkhole opened in an unused portion of the zoo.

It is about 50 yards by 85 yards and about 50 feet deep in spots.

Much of the zoo sits over Louisville Mega Cavern, which has been closed because of a small earthquake.

The Louisville Zoo remained closed Thursday while investigators work to learn what caused a football-field-sized sinkhole to open in an undeveloped part of the zoo.

Zoo staff found the sinkhole about 8:30 a.m. Wednesday during a routine walk-through, zoo spokeswoman Kyle Shepherd said. No people or animals were hurt, and no buildings were damaged, Shepherd said.

"Right now, our priority is safety," Louisville Zoo Director John Walczak said. "So we're going to let all the engineers do their work, and we'll make that determination (to open) when they say it's safe."

Officials estimate the sinkhole is about 50 yards by 85 yards and about 50 feet deep in spots, Louisville Metro Emergency Services Director Jody Meiman told reporters.

(MORE: Winter 2018-19 Was Wettest on Record in U.S., NOAA Says)

Meiman said rain runoff or "natural occurrences" underground could have caused the sinkhole , the Louisville Courier Journal reported, but authorities were still investigating the official cause.

A wet pattern has recently been in place in the Louisville area, resulting in rounds of rain and some snow. Louisville has measured 11.74 inches of precipitation since the start of the year, which is more than 4.5 inches above average, weather.com meteorologist Linda Lam says.

Louisville experienced its seventh wettest-February on record with 8.38 inches. This wet start to the year comes just after the wettest year on record in Louisville, 2018, which saw the wettest February and September on record with more than 10 inches of rainfall in both of those months.

About 70 percent of the zoo, including part of the sinkhole, sits over the Louisville Mega Cavern, a former limestone mine that is now, among other things, a major tourist attraction. It includes ziplining, rope walks and a mountain bike trail.

The Mega Cavern also remained closed Thursday because of a 3.4 magnitude earthquake reported Tuesday in neighboring Tennessee.

It's standard procedure for the cavern to close if seismic activity is detected, the Courier Journal reported.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/KYsinkhole1.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/KYsinkhole1.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/KYsinkhole1.jpg?v=ap&w=980&h=551 800w" > Officials stand on one edge of a giant sinkhole on the property of the Louisville Zoo, on Wednesday, March 6, 2019, in Louisville, Kentucky. Louisville Zoo spokeswoman Kyle Shepherd told news outlets the sinkhole was found Wednesday in an undeveloped area away from any zoo animals. She said no people or animals were reported injured and no buildings damaged. (Jeff Faughender/Courier Journal via AP)

Charles Park, the cavern's executive vice president, said in a news release, “Earthquakes happen. This is a natural event. It is the Louisville Mega Cavern’s standard customary practice to close the cavern in the event of an earthquake. Our mine engineers will be inspecting the cavern out of an abundance of caution for the general public and our tenants."

Meiman said one of the engineers inspecting the sinkhole doesn't think it was caused by the earthquake, which happened about 4 p.m. Tuesday. Officials think the sinkhole formed between late Tuesday and early Wednesday morning.

The officials examining the sinkhole include geo-technical and mining engineers and employees from the Louisville Water, Metropolitan Sewer District, Louisville Fire and Louisville Gas & Electric.

Meiman said nearby commercial and residential are safe.