The bodies of the victims – Prathomporn Siriwat and Pratchawat Wasu-anan, both 24 and of Chiang Mai – were taken to Mae Tang Hospital in Mae Tang district.

Wongpan Wongsamut, a chief engineer of the provincial irrigation office said that the accident took place after an explosion at a side tunnel, which is part of the Mae Ngad-Mae Kuang Water-Diverting Tunnel.

He said that both geologists were sent into the tunnel to examine the layers of stone when the loosened rock fell on them at about 7am.

“We are investigating the cause of the accident,” Wongpan said.

The project, at Tambon Mae Horphra, is now sealed pending the probe. It is part of a Bt11.5 billion 23-kilometre tunnel project which is slated to be completed by 2021. Both men were working for an Italian-Thai company on the site.

During a press conference Thursday, a construction expert from the project’s advisory company Noppadol Pianwet explained that there had been a controlled explosion on Tuesday at the section of tunnel where the accident happened and it was supported by cement and bracing equipment.

The two geologists were sent to conduct an inspection for a further tunnel drilling process but a large rock, measuring about three-metres by one-metre, fell on them before smashed on the tunnel floor, he said.

He dismissed speculation that the tunnel collapsed or the rocks fell due to water pressure.