WEBVTT 00:00:00.411 --> 00:00:03.911 Russia's Kola Peninsula, 1992. 00:00:03.986 --> 00:00:10.286 Geophysicist David Smythe takes a video camera onto the rig floor of the Kola Superdeep Borehole. 00:00:10.361 --> 00:00:13.761 Workers handle pipe beneath a forest of steel. 00:00:13.886 --> 00:00:18.986 Smythe identifies the operation as an attempt to retrieve dropped drill pipe. 00:00:19.061 --> 00:00:23.611 This is a hole that has already gone more than twelve kilometres into the Earth. 00:00:23.886 --> 00:00:26.781 And the job in front of the crew is getting equipment back. 00:00:27.297 --> 00:00:31.072 That is where the story of superdeep drilling gets interesting. 00:00:31.147 --> 00:00:33.572 What did people go down there to find? 00:00:33.622 --> 00:00:35.722 What did the rock actually do? 00:00:36.007 --> 00:00:42.257 And what finally made the next metre too difficult, too expensive, or no longer necessary? 00:00:42.332 --> 00:00:49.521 This is a tour of some of the major holes that tested those limits, from scientific experiments to the search for deep gas. 00:00:50.224 --> 00:00:52.114 First, one distinction. 00:00:52.214 --> 00:00:55.684 Measured depth follows the hole, including its bends. 00:00:55.784 --> 00:00:58.909 Vertical depth measures how far down you have gone. 00:00:59.164 --> 00:01:02.089 A long well can travel mostly sideways. 00:01:02.164 --> 00:01:06.614 At sea, the water above the seabed is another separate measurement. 00:01:06.664 --> 00:01:13.271 We'll keep those quantities separate, and use reported drilling depths without turning them into a misleading league table. 00:01:14.059 --> 00:01:17.259 Before Kola, there was Project Mohole. 00:01:17.514 --> 00:01:22.389 American researchers wanted to drill through the oceanic crust toward the mantle. 00:01:22.584 --> 00:01:28.959 The ocean offered a tempting advantage: the crust there is much thinner than beneath the continents. 00:01:29.034 --> 00:01:33.031 It also put kilometres of moving water between the rig and the rock. 00:01:33.596 --> 00:01:40.271 In 1961, a converted drilling barge worked near Guadalupe Island, off Mexico. 00:01:40.346 --> 00:01:48.436 The deepest test penetrated about a hundred and eighty-three metres beneath the seabed, below roughly three and a half kilometres of water. 00:01:48.511 --> 00:01:50.186 It recovered basalt. 00:01:50.261 --> 00:01:58.025 The achievement was learning to drill from a floating platform in deep water, with the much deeper scientific objective still ahead. 00:01:58.654 --> 00:02:03.354 Holding position, losing equipment and keeping holes open were difficult. 00:02:03.454 --> 00:02:06.529 Costs and management became problems too. 00:02:06.604 --> 00:02:10.129 Congress ended funding in 1966. 00:02:10.439 --> 00:02:13.414 Mohole never reached its intended boundary. 00:02:13.514 --> 00:02:19.876 Here, the stopping point was set above ground, before the central geological question could be answered. 00:02:20.662 --> 00:02:28.062 Kola began in 1970, in the Soviet Union, with an ambitious fifteen-kilometre design. 00:02:28.137 --> 00:02:31.362 The site put ancient crust within reach of the drill. 00:02:31.437 --> 00:02:37.212 Researchers wanted physical samples and measurements to test their interpretation of the deep Earth. 00:02:37.287 --> 00:02:42.936 They also wanted to understand the mineral potential of the nickel-bearing Pechenga region. 00:02:43.512 --> 00:02:52.512 The hole passed through a thick volcanic and sedimentary sequence, then into older metamorphic basement below about six point eight kilometres. 00:02:52.612 --> 00:02:59.912 Gneiss, amphibolite and other crystalline rocks complicated the simple picture inferred from seismic waves. 00:03:00.197 --> 00:03:07.255 A change in wave speed did not necessarily mark the neat granite-to-basalt boundary that researchers had expected. 00:03:07.836 --> 00:03:11.886 That matters because a seismic section is an interpretation. 00:03:11.986 --> 00:03:14.761 Drilling gives you a sample to test it against. 00:03:14.861 --> 00:03:20.661 Rock composition, deformation and fluids in fractures can all affect the signal. 00:03:20.736 --> 00:03:26.736 Kola helped show how much information was hidden inside what had looked like a straightforward boundary. 00:03:26.811 --> 00:03:32.367 The deeper crust contained a more complicated combination of rocks and fluid-bearing structures. 00:03:32.992 --> 00:03:35.942 The engineering assumptions were being tested too. 00:03:36.017 --> 00:03:38.567 Temperatures rose more than expected. 00:03:38.642 --> 00:03:44.027 The deeper rock was difficult to keep open, and equipment became stuck or broke. 00:03:44.102 --> 00:03:53.157 After one major accident in 1984, the researchers' retrospective describes kilometres of drill string left in the hole. 00:03:53.232 --> 00:03:57.817 Reaching a record once did not mean the crew could simply continue downward from it. 00:03:58.352 --> 00:04:02.127 When equipment is lost downhole, drillers call it a fish. 00:04:02.202 --> 00:04:05.302 They try to catch it with another tool and pull it free. 00:04:05.377 --> 00:04:10.452 If that fails, they may plug back and steer a new branch around the obstruction. 00:04:10.527 --> 00:04:14.552 The rig can spend days working without making the well any deeper. 00:04:14.627 --> 00:04:19.453 A depth record hides all those metres drilled, abandoned and drilled again. 00:04:20.100 --> 00:04:27.020 That brings us back to Smythe's camera in 1992, and the attempt to retrieve dropped pipe. 00:04:27.120 --> 00:04:33.885 Here is David Smythe recalling the operation for BBC Witness History in 2024. 00:04:34.320 --> 00:04:40.240 [David Smythe, recorded 2024] And before you can do any more drilling, you had to recover that. They were doing what they call a fishing expedition. 00:04:40.752 --> 00:04:45.527 Kola's famous reported depth is about twelve point two six kilometres. 00:04:45.602 --> 00:04:48.827 It stopped short of the fifteen-kilometre design. 00:04:48.902 --> 00:04:56.877 The account written by Kola researchers places the end of drilling in 1992, amid a drastic loss of funding. 00:04:56.977 --> 00:05:06.255 Heat, unstable ground and repeated recovery work had made progress difficult; the financial collapse then removed the ability to keep trying. 00:05:06.829 --> 00:05:10.854 But the end of deepening was not the end of every investigation. 00:05:10.929 --> 00:05:18.629 German research records document a Kola hydraulic project funded from 1998 to 2001. 00:05:18.704 --> 00:05:23.554 The site was later mothballed and its drilling infrastructure dismantled. 00:05:23.629 --> 00:05:29.063 The end of drilling, the later research and the closure of the working site were separate chapters. 00:05:29.891 --> 00:05:35.666 Now to Oklahoma's Anadarko Basin, where the objective was commercial hydrocarbons. 00:05:35.741 --> 00:05:40.091 The Baden well passed thirty thousand feet in 1972. 00:05:40.166 --> 00:05:44.691 Its deep Hunton test did not establish the hoped-for commercial result. 00:05:44.791 --> 00:05:49.691 Bertha Rogers followed, with the Hunton dolomite again a major target. 00:05:49.791 --> 00:05:55.955 These were expensive attempts to find a working reservoir beneath a very thick stack of sedimentary rock. 00:05:56.574 --> 00:05:59.074 The drilling fluid had a difficult job. 00:05:59.149 --> 00:06:05.324 It needed enough pressure to hold back deep gas, while other formations could take that fluid away. 00:06:05.424 --> 00:06:08.174 Unstable shales could fall into the hole. 00:06:08.249 --> 00:06:10.499 Heat changed the mud's behaviour. 00:06:10.574 --> 00:06:14.724 The operational account describes constant testing and adjustment. 00:06:14.939 --> 00:06:20.281 The challenge was managing the whole open hole, not just cutting the rock at the bottom. 00:06:20.836 --> 00:06:32.136 In 1974, Bertha Rogers reached a reported total depth of thirty-one thousand, four hundred and forty-one feet: about nine point five eight kilometres. 00:06:32.211 --> 00:06:37.561 Near the bottom, high pressure and sulfur-bearing fluids complicated the ending. 00:06:37.636 --> 00:06:45.951 The United States Geological Survey records sulfur crystals coming to the surface after the overpressured well was brought under control. 00:06:46.512 --> 00:06:50.387 The deepest rocks did not become Bertha's producing reservoir. 00:06:50.462 --> 00:06:54.662 Gas production came from the much shallower Atokan granite wash. 00:06:54.737 --> 00:06:59.237 Baden also had shallower completion work after reaching total depth. 00:06:59.337 --> 00:07:04.112 Stopping the downward advance could be followed by another phase of the well's life. 00:07:04.187 --> 00:07:11.069 A depth record, a successful geological test and a productive completion are three different measures of success. 00:07:11.891 --> 00:07:19.391 In Bavaria, Germany's Continental Deep Drilling Programme, KTB, returned to the scientific questions. 00:07:19.466 --> 00:07:21.091 How hot is the crust? 00:07:21.141 --> 00:07:22.841 How much stress does it carry? 00:07:22.891 --> 00:07:24.641 Where can fluids move? 00:07:24.691 --> 00:07:29.616 The site lay in metamorphic rocks shaped by an ancient continental collision. 00:07:29.831 --> 00:07:35.876 The team drilled a four-kilometre pilot hole before starting the main borehole in 1990. 00:07:36.454 --> 00:07:39.629 The pilot supplied core and practical warnings. 00:07:39.729 --> 00:07:44.179 Steeply tilted rock fabric made a straight hole difficult to maintain. 00:07:44.229 --> 00:07:47.854 Under stress, parts of the borehole wall broke away. 00:07:47.929 --> 00:07:52.054 In the main hole, deeper sections also narrowed with time. 00:07:52.129 --> 00:07:59.346 Researchers identified this convergence as a particularly serious obstacle below about seven and a half kilometres. 00:07:59.999 --> 00:08:02.549 Think about the clearance around the drill pipe. 00:08:02.624 --> 00:08:07.199 It only has to disappear in one troublesome section to trap the assembly. 00:08:07.454 --> 00:08:09.704 A harder bit cannot solve that. 00:08:09.779 --> 00:08:19.204 The crew needs a hole that stays open, drilling fluid that works with the formation, and enough room to install protective casing or a liner. 00:08:19.279 --> 00:08:22.349 Each recovery operation consumes time and money. 00:08:22.977 --> 00:08:31.127 At about seven and a half kilometres, temperature exceeded the tolerance of the electronics in KTB's automatic vertical-drilling system. 00:08:31.202 --> 00:08:34.302 The near-vertical path became harder to maintain. 00:08:34.402 --> 00:08:39.552 Stuck assemblies and unstable sections forced repeated liners and sidetracks. 00:08:39.627 --> 00:08:44.477 The contemporary operational account describes the remaining funds running low. 00:08:44.552 --> 00:08:51.242 In October 1994, deepening ended at a reported nine thousand, one hundred and one metres. 00:08:51.894 --> 00:08:56.769 The well recorded more than two hundred and fifty degrees Celsius near its bottom. 00:08:57.029 --> 00:09:03.129 But saying that heat alone stopped KTB leaves out the failing hole and the budget. 00:09:03.204 --> 00:09:05.754 A drilling system has many limits. 00:09:05.829 --> 00:09:09.236 The first one you meet can make the others more expensive to manage. 00:09:09.832 --> 00:09:13.132 KTB's afterlife is part of its achievement. 00:09:13.207 --> 00:09:16.007 The boreholes became a deep laboratory. 00:09:16.082 --> 00:09:22.782 The cores remained available for new questions, and experiments could examine stresses and fluids in place. 00:09:22.882 --> 00:09:26.557 Ending the drilling phase preserved a scientific asset. 00:09:26.822 --> 00:09:31.741 A useful hole can keep producing knowledge long after it stops producing new metres. 00:09:32.564 --> 00:09:34.864 The story continues in China. 00:09:34.939 --> 00:09:44.539 In the Tarim Basin, Shendi Taqe One combined scientific exploration, oil and gas questions, and a test of deep-drilling equipment. 00:09:44.639 --> 00:09:52.135 Drilling finished on January fifth, 2025, at a reported ten thousand, nine hundred and ten metres. 00:09:52.777 --> 00:09:54.777 The rock was part of the problem. 00:09:54.852 --> 00:09:58.502 Hard dolomite with chert inclusions damaged the bits. 00:09:58.577 --> 00:10:05.622 Crews also battled drill-string fatigue, unstable intervals and drilling fluid escaping into the rock. 00:10:05.697 --> 00:10:11.772 The operator's account says the last nine hundred and ten metres took more than three hundred days. 00:10:11.847 --> 00:10:15.558 The deepest stretch consumed a disproportionate share of the effort. 00:10:16.156 --> 00:10:17.431 Why stop there? 00:10:17.481 --> 00:10:20.781 The drill had reached ancient Cambrian source rocks. 00:10:20.831 --> 00:10:27.431 The operator said the required scientific information had been gathered and hydrocarbon shows had been found. 00:10:27.506 --> 00:10:29.481 Going deeper added risk. 00:10:29.556 --> 00:10:34.356 The decision was to end deepening and move into testing and further research. 00:10:34.431 --> 00:10:39.621 A hydrocarbon show still needs testing before anyone can call it a commercial discovery. 00:10:40.236 --> 00:10:51.661 A newer research paper reports Xin'an One, in the Junggar Basin, completed in October 2025 at eleven thousand, one hundred and fifty-six metres total depth. 00:10:51.736 --> 00:10:54.411 It tested deeply buried clastic rocks. 00:10:54.486 --> 00:11:01.571 Reservoir quality was generally poor, although the team identified more promising intervals and oil and gas shows. 00:11:01.646 --> 00:11:06.221 The gas analysis pointed to highly mature, coal-derived gas. 00:11:06.296 --> 00:11:09.457 Testing was still being planned in 2026. 00:11:10.101 --> 00:11:17.491 And Shendi Chuanke One, in Sichuan, passed ten thousand metres in October 2025. 00:11:17.566 --> 00:11:22.116 Its goals include deep petroleum geology and equipment development. 00:11:22.191 --> 00:11:29.516 By our November 2026 cutoff, that is the latest confirmed milestone in the operator's account. 00:11:29.591 --> 00:11:33.841 A final depth and outcome have yet to be established in that record. 00:11:33.916 --> 00:11:35.970 This part of the story remains open. 00:11:36.746 --> 00:11:40.821 These projects did not encounter a single universal depth limit. 00:11:40.921 --> 00:11:42.846 Mohole lost its funding. 00:11:42.921 --> 00:11:47.271 Kola combined difficult drilling with the loss of financial support. 00:11:47.346 --> 00:11:50.971 Oklahoma's wells could seek production higher in the section. 00:11:51.046 --> 00:11:57.696 KTB stopped amid hole instability and dwindling funds, then continued as a laboratory. 00:11:57.771 --> 00:12:02.749 Taqe's team judged that the next stage should be testing what the well had already reached. 00:12:03.355 --> 00:12:11.560 The useful question is what we need from the next metre, whether we can keep the hole usable, and whether the answer is worth the work. 00:12:11.635 --> 00:12:14.445 This is GeoNavigate Industry Chatter.