I know that a lot of iridium can be found in the lower levels of the Skull Cave. The lower you go, the more will spawn on the floor. But how do I descend the caves fast? Since the bus doesn't operate until 10am everyday, and the longest you can go before becoming unconscious is until 2am, there are effectively ~15.5 hours of time to explore the cave, once you factor in having to walk to it. Realistically, I would say there is about 14.5 hours to explore, if you would like to make it out without going unconscious and potentially losing items.
I know there are holes one can fall down to advance a certain amount of floors, but this isn't that effective since finding the holes is pretty much random and it damages you. Like the other mines, you can find ladders that take you down a level, but this is also slow since you need to keep mining to find them, or kill enemies. Is there a fast and effective way to descend?