A special character of these mounds [the Kess-Kess carbonate mud mounds] is the numerous Neptunian dikes. These are fracture systems, which have been filled with other sediments and minerals. It is suspected that these dikes represent spring conduits (seeps) during the formation of the mounds, and that the seeping fluids were of a hydrothermal nature ….

Styx is a fountain of Arcadia, whose waters are so deadly that they presently kill whatsoever drinks thereof; so corrodiating, that they can only be contained in the hoof of a mule.

Admittedly the scenario that it was an unholy alliance of numerous enemies of the Doctor and other alien races that had united to create the trap was rather fanboyish on Moffat's part, even though it was great to see them assembled together like that and showed just what a threat the universe was facing once such an alliance had been created.

. . . but too much security on the republican part will give time to his talents and indefatigableness to extricate them.