For Rudi Wolf, from Germany, his experience as technical consultant for the rescue team at the site of the collapsed gypsum mine in Pingyi county, Shandong province in China, was most tiring and rewarding.
"The job here has been the most difficult one in my life," Wolf said.
Wolf, 55, who works for Prakla, a Germany company that specializes in drilling technologies, has about 35 years' working experience and has helped in similar rescue operations in other countries such as Germany and Chile, he said. "The reason that the rescue operation took so long is we have very, very bad ground here," Wolf said.
Wolf said the underground space of the site of the collapse is extremely complex and unstable, making the drilling operations to rescue the four survivors very difficult.
The underground space contains different layers made of materials such as claystone, gravel and limestone with caves, and "is hard and then it gets softer, and then it gets hard again and then soft again".
The drilling teams had to change drilling methods constantly to cope with the complex underground geology and frequent breakdowns, he said.
The rescue team has changed the drilling methods more than 20 times, adopting various methods such as air hammer drilling, reverse circulation air drilling and reverse circulation mud drilling, until the pipes reached a depth of more than 200 meters to connect to the area where the four miners were living, Wolf said.
And there was also a layer with water underground, so rescuers had to think of ways to seal the pipe sent down to save the survivors so water will not seep into the pipe, he said.
Fighting against time to save the survivors, Wolf, like hundreds of others, has been extremely busy since the start of the operation.
He has been working for about 15 hours every day, seven days a week in the past five weeks, and he said he worked for 38 hours in a row when he first arrived at the site.
"I spent New Year Day on a night shift," he said.
When the four survivors were lifted out after being trapped for 36 days on Friday night, he was also very happy and excited and joined in the applause with other rescuers on site, he said.
Now the operation has been successful, Wolf said he is eager to go back to his home in Germany and "take my family into my arms", although he still does not know when he can leave.
Wolf said the Chinese government provided good support for the rescue, and he worked well with Chinese colleagues, including an official surnamed Wang, who is the chief commander of the rescue team.
"He is a very good, smart man and took the right decisions. I must give him my compliments," he said.
The working experience has also been quite a rewarding one for Wolf, who is also a trainer of drilling technologies.
"I will show everything to my German students. I learned a lot here," he said.