BASTROP, Texas− Three people from Florida fell into an underground water reservoir in Central Texas and died on Wednesday after trying to catch a dog that had fallen into the hole.

Authorities said the group was going to meet a friend to go hog hunting in Bastrop County, just outside of Austin. A dog escaped from their truck and fell in the reservoir, prompting three of the people to go after the dog.

"It's a sad day whenever it ends like it did yesterday," Bastrop County Sheriff Maurice Cook said during a news briefing Thursday. "Can you imagine? You have loved ones and they come to Texas to hunt and then it ends like this, which was a sad tragedy that just happened."

The hole in the ground was a cistern, which is an underground water reservoir common in rural areas that is used to supply water to homes. The cistern's opening was ground level and left open, but typically those holes should be covered, Cook said.

He said the cistern was essentially a "hole in the middle of a cornfield."

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Hydrogen sulfide fumes found in cistern

Authorities smelled fumes coming from the hole and found high levels of hydrogen sulfide, a toxic gas common in septic tanks and wells. Cook said they speculate the gas affected the buoyancy of the water and caused them to not be able to float.

"It wasn't long in the tank before they went to the bottom," he said.

Officials said two men and a woman were recovered from the hole: Delvys Garcia, 37; Denise Martinez, 26; and Noel Vigil-Benitez, 45.

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A hog hunt among friends

Cook said his office received a call at 1:02 a.m. that three people had fallen into a hole near County Road 461 in Elgin. Cook said the call came from the fourth person in the group, a friend who lives in the area and invited the other three to hunt hogs. He did not release the name of that person.

From the initial investigation, Cook said, it appears that the group was about to start hunting when one of the dogs escaped from the truck. This prompted the group to start searching for the dog, when they found that the dog fell into this hole. One of the hunters jumped into the cistern to try to rescue it.

Cook said there was about a 6- to 8-foot gap between the water's surface and the cistern's opening.

Diving teams were hesitant to get in the water due to the gas, and after contacting six different diving teams, all of whom declined to get in, they decided to pump out the water and ventilate out the gas. They then flew a drone into the hole to check the integrity of the walls.

Once this was done, Cook said a deputy went into the hole three separate times, starting at about 6:30 p.m., and took out the three bodies and the dog. All of the bodies were sent to the Travis County medical examiner's office. The rescue efforts ended at about 7:30 p.m.

Cook said the cistern was on private property and preliminary information indicates that the property owner recently died. Authorities did not release the name of the property owner.

Cook said each of the friends went in to rescue one another. He said it appears one man jumped in immediately, and the other man and the woman took off their shoes and other pieces of clothing before jumping into the water.

"That group was there to save each other," he said. "That takes a lot of guts to jump in."