The Serps Pump Chamber seems to be similar to the huge Waterfall at the d/s end of Rubix... 9/10 times both are inaccessible, but as i with Zero finding lower Rubix drained and empty the first two times he explored it, i was lucky enough to visit Serps when the chamber and its approach tunnel were only knee deep. Never had any luck with Rubix tho, been full to the ceiling all 5 times i visited. In the case of Rubix, it drains to a deeplevel tunnel that connects the d/s end of Rubix to the deep tanks off the side of the Ranelagh n KSP sewers. We once got into it, but being new, it had no easylifts n we had to block off a street in West London to get the lid open. Otter n JD werent interested in traversing it given the disruption to traffic and the fact its concrete... i was interested in it as its the only sewer tunnel in London that crosses beneath the Thames.