Speaking with Tuoi Tre (Youth) newspaper the same afternoon, a representative of the Hanoi Metropolitan Railway Management Board (MRB) confirmed the incident and stated that meetings were ongoing with relevant units to identify the cause and determine solutions to prevent similar occurrences in the future.

Detailed information on the incident is pending a report from the contractor and will be shared with the media, the representative said.

According to MRB, upon detecting the incident, the contractor immediately activated the project's emergency response protocol.

MRB also ordered a halt to the drilling activities to reduce the amount of mud being overflowed.

The contractor has mobilized maximum manpower, deployed mud vacuum trucks, and carried out mud collection and on-site treatment, while re-checking geological conditions in the affected area to stabilize the situation quickly, MRB stated.

At 4:00 pm on February 20, tunneling additives also erupted to the surface on Alley No. 7, Giang Van Minh Street in Ba Dinh District during the excavation process of the same project.

MRB, consultants, and the contractor identified the initial cause as possibly being due to the presence of underground wells or old drainage systems, which created pathways for the tunneling additives to escape to the surface.

Consultants and contractors had surveyed the structures along the route before beginning tunneling operations. However, due to changes in property ownership over time, some data was incomplete, resulting in missing information about underground structures such as household wells, boreholes, or geological survey drill holes.

The phenomenon only lasts briefly and is a common occurrence in urban tunneling projects, MRB explained.

Le Thi Lam, a resident on Cat Linh Street, told Tuoi Tre that on Monday morning, the contractor told her family that they needed to vacate their home for two days for the tunneling work of the Nhon - Hanoi Station metro project.

"This afternoon, mud surfaced in front of my house. It rose nearly one meter high before sandbags were used to block it. Even after that, the mud continued to rise," Lam said.

Lam expressed concern about the tunneling taking place directly beneath her house and the possible impact of the mud eruption on the building's structure.

"This is a national project, and we support it, but it must be done in a way that does not affect people's lives, rights, or homes," she added.

The Nhon - Hanoi Station metro line runs 12.5 kilometers from Nhon in Nam Tu Liem District to the Hanoi Railway Station in the downtown area, with 8.5 kilometers on elevated tracks, four kilometers underground, and one depot.

Work on the metro line began on September 25, 2010 and its completion was initially scheduled for 2016.

However, the project was delayed several times for various reasons, increasing its total investment to VND34.5 trillion (US$1.4 billion) from VND18 trillion ($712 million).

The underground section is expected to be finished by December 2027, VnExpress reported.