You have discovered a pit of unknown depth with impervious smooth walls. You also have a 1/2" twisted nylon rope which is 5,000km long and marked every meter. It is anchored to the ground by an attachment point that can bear the full weight of the rope, which feeds the rope through it and down into the pit (an automatic winch). The attachment point also has a sensitive force gauge built in, so your plan to measure the depth of the pit is to feed rope into the pit until the weight stops increasing, which will indicate that you've hit the bottom. The rope can be fed through the winch at a maximum 5km/h, and does not stretch.
How deep of a pit are you able to measure with this apparatus? Is the only limit the length of your rope?
There is another limit. Why?