The dam was built with an enormous concrete block at its base. A small passageway runs through it, reachable from the dam's machine room. It was in this passageway, the evidence suggests, that an explosive charge detonated and destroyed the dam.
At 2:35 a.m and 2:54 a.m. on June 6, seismic sensors in Ukraine and Romania detected the telltale signs of large explosions. Witnesses in the area heard large blasts between roughly 2:15 a.m. and 3 a.m. And just before the dam gave way, American intelligence satellites captured infrared heat signals that also indicated an explosion.
As the water levels further dropped this week, they fell below the top of the concrete foundation. The section that collapsed was not visible above the water line — strong evidence that the foundation had suffered structural damage, engineers said.
Engineers cautioned that only a full examination of the dam after the water drains from the reservoir can determine the precise sequence of events leading to the destruction. Erosion from water cascading through the gates could have led to a failure if the dam were poorly designed, or the concrete was substandard, but engineers called that unlikely.
Ihor Strelets, an engineer who served as the deputy head of water resources for the Dnipro River from 2005 until 2018, said that as a Cold War construction project, the dam's foundation was designed to withstand almost any kind of external attack. Mr. Strelets said he, too, had concluded that an explosion within the gallery destroyed part of the concrete structure, and that other sections then were torn away by the force of the water.
The seismic signals were picked up on two sensors, one in Romania and one in Ukraine, and occurred at 2:35 a.m. and 2:54 a.m. Ukraine time, said Ben Dando, a seismologist at Norsar, a Norwegian organization that specializes in seismology and seismic monitoring.
The signals were both consistent with an explosion, Dr. Dando said — and not, say, the collapse of the dam on its own.
He said that the network could determine the time of an explosion to within a couple of seconds, but that the location of the blasts was less certain. For example, Norsar could locate the 2:54 a.m. signal to have originated within a zone 20 or 30 kilometers across that included the dam.
Professor Baecher said it was possible, though unlikely, that water flow from the damaged gates somehow undermined the concrete structure where it sat on the riverbed. But he said an examination of the drawings indicated that the design had protected against that possibility with standard measures. One of those is a so-called "apron" of concrete on top of the riverbed to the downstream side of the dam.
"This appears to be a well-engineered dam of modern design," he said.
Neither this previous damage, nor the pressure caused by the high water level or the static position of the cranes is likely to have caused the collapse of the dam's concrete foundation, experts said, unless the concrete was of low quality and already prone to deteriorate. The large flows would also be insufficient to undermine the dam's foundation unless, for some reason, the concrete apron — the downstream cover placed over the river bottom — contained flaws or the soil was much softer than accounted for in the design.
A video that emerged this week, after water levels had dropped, provides clear evidence of the catastrophic failure. It shows that the top of the concrete foundation, not just the gates, was destroyed.