Large diameter drilled shaft foundations have increased in popularity ever since the early 1990s and the widespread implementation of more rigorous seismic design criteria. They're commonly used in pier foundations for major river bridges: They bore a deep shaft down through the the soil to bedrock, then use a rock bit to construct a socket into the rock. The shaft and socket are then inspected and cleaned, a massive rebar cage is lowered into the shaft and the void filled with concrete. Uncertain what went wrong with this one, but it appears they've bored out ALL of the concrete in the center of the pier foundation all the way to the bearing end of the pier. Most likely there was a void or solution cavity immediately below the socket bottom that went unnoticed during construction, and a failure was noted during load testing - or maybe settlement occured that made them go WTF??? What you're looking at near the top is series of 4-6” drill holes used to remove all of the concrete inside the rebar cage. They angle the outer holes to the outside so there's room to get the cutting head close to the rebar cage in the next deeper level of coring. Near the bottom, you can see where all the concrete has been removed right up the the rebar cage - this is the more stable part where it's socketed in rock. But all that water in the bottom of the hole is freaky. Remember - this is likely adjacent to or in a big river. Wild guess: Pore water pressure at the top of that rock socket is likely 30 or 40 psi. Guy I went to college with was a construction inspector for the DOT here. He's been down in the bottom of excavations like these inspecting cleanliness and bottom integrity prior to rebar and concrete placement. He loved it - sent pics several times. And he just got standard pay. He loved to go caving back in the day too - he's just wired different. Some would say he's nuts. No way you ever would have caught me in something like that.