Doctor Who question, re: finale; what does the medusa cascade refer to?

there’s a part in the finale where the master goes something along the lines “he sealed the rift at the Medusa Cascade single-handed”.

Now the classic series were a little before my time so i don’t really know the reference and have been poking around search engines and wikipedia looking for what the Medusa Cascade refers to, but have yet to find anything. Could someone tell me?

There’s a very good chance that this reference was either made up, or was referencing something that took place during one of the many books written after the classic series ended. Perhaps one of the Eighth Doctor’s Adventures, I’m not really sure. I’ve been looking for the answer myself…