On 26 May 2013 three people were drowned in Tham Pung, amphoe Muang, Mukdahan province in north-eastern Thailand. This region of Thailand does not have any limestone and from the news photos Tham Pung appears to be formed in sandstone.

The incident made the Thai TV news and newspapers, but not the English language press.
From Hunsa.com (translated with a lot of help from thai2english)

 
At 13:30 on 27th May Police Lieutenant Suradet Bunphithakchai, duty officer at Muang Mukdahan Police Station, received information that people had been drowned in the polluted water that is inside inside Tham Pung (Stomache Cave). They were villagers from moo 12, Ban Na Do, Na Sok, Muang Mukdahan. Going to check this report were Police Major General Surachai Khuandechakhup, Commander of Mukdahan Provincial Police, Major Ukkrit Songchaisanguan, chief of police and a doctor from Mukdahan hosptial. The scene is in a valley with extensive forest a distance of about 3km from the village. The personnel had to walk, with a rocky shortcut, about 500m to the cave entrance area where they found the dead body of Mr Ngoen Chathipha Ayu, 59 years old. The pale corpse was investigated but no marks or scratches were found to indicte that he had been attacked.The investigation was started knowing that before having the accident Mr Ngoen was with 2 friends who were his son Mr Sa-Ngop Ansaen, aged 39 years and Mr Thonchot Linto Ayu, 48 years old. Together with a total of 9 relatives they had gone in to the forest seek herbs and stay overnight near the rocks with Tham Pung. It is believed that the cave has metal amulets (lek lai) on the 26 May. When they finished looking for herbs Mr Ngoen, Mr Sa-Ngop and Mr Thonchot persuades each other to dive to enter the cave zone in order to search for the lek lai reported to be there by local custom and consequently disappeared all night long until the morning when the many relatives feared that an accident had happened so they left to get people from the village Phop Phra Chai Chana of the renowned Wat Pu Sang to help. They twice dived to a depth of approximately 2 metres bringing the still and lifeless corpses of Mr Ngoen and Mr Sa-Ngop, which were submerged in the polluted water. After realising that Mr Thonchot had not yet been found the Tambon officials therefore gave the order to quickly bring equipment to search for the lost person.
 
According to Mr Sa-lat Ukam, the village headman, it is thought that Tham Pung is a cave that goes deep down into the underground. Inside the cave there are rocks and a large-sized chamber that would hold no less than 50 people. When it is the dry season villagers enter the cave searching for frogs which are very abundant. However, when it is the rainy season it is closed by floods and the villagers do not dare enter the cave as they are scared of the dangers. Also, the villagers believe that inside the cave there are lek lai because someone once saw them, but now there isn't anyone bold enough to go there as there are scared of the mystery and to suffer death. In this occurance it is assumed that the deceased and his 2 friends, before going inside to look for the lek lai, in accordance with widely spoken opinion performed a ceremony and lit joss sticks and candles to ask for foregiveness from the guardian spirit, t was observed that there were the remains of joss sticks and candles which are lit to worship in the cave mouth area, before then diving to duck through to enter the inside of the cave. But because noone person in the district now knows the dangers inside the cave it is consequently risky to enter and has resulted in giving up 2 lives and the disappearance of 1 person. As the fate of the missing person was still unknown, either still alive or already dead the order was given by the Mukdahan police chief to fully assist by all means available to find professional divers in the district and neighbouring provinces to give help and search teams to then go inside the cave. Most recently in the evening of the same day a professional diver brought out the dead corpse 1 more person being Mr Sa-Ngop Ansaen. The remaining missing body is Mr Thonchot Linto who was still being searched for, but not found. Because it was the evening and starting to get dark and visibility was becoming unclear the professional diver stopped the search on this occasion and will start again the next day.
 
It is also reported at Khao Sod, Daily News and Paidoo.