Imagine changing the inserts on a facemill from the inside of the cutter body. I had the privilege of crawling around Bertha's guts spread out on the ground during seal damage repairs, while she was mining, and after she broke through into the recovery pit. Bertha started mining using disk cutters (the wheel-like tools) to get through the concrete headwall and grouted ground near the start. Later, the disc cutters were swapped for "precutting bits". Only a fraction of Bertha's cutters were changeable from the "comfort" of the interior of the spokes. Any other cutter changes required emptying the area behind and around the cutterhead, making space in front, and doing a hyperbaric intervention. Bertha's "chip augers" were about 5' diameter and just absolutely massively heavy corkscrews. She started out equipped with 24 750-hp VFD-driven motors to drive the cutterhead through a 30-ft-diameter slewing ring bearing.
Bertha was stunningly massive to see it in person. The modular lift tower and crawler cranes needed to do the repairs were no slouches, either. Working on that project has been one of the highlights of my career.