Several years ago, I went diving in Lake Silencio, Utah. I travelled for a day to get to the scenic Glen Canyon Reservoir where the lake is located. I hired SCUBA kit and headed to a corner of Wahweap canyon, on the opposite side of the canyon from where the Doctor meets his end.
I descended into the clear warm water to the thermocline. It's about ten meters below the surface, a shimmering meniscus with tiny organisms gathering in a thin layer where the colder water underneath meets the summer warmth above. I'm still not sure why they don't mix. Beneath there the warming red rays of the sun fade out and everything looks blue and gray.
I found an old sunken boat hull and quite a lot of fish. The fish are stocked; few natural populations can survive in the drowned waters of Glen Canyon today. Before the reservoir, it was the greenest and liveliest place in the region. I suppose it's a natural metaphor that our long lived friend will pass there like the community of nature passed out of existence in the 1960s as the rising waters drowned it.
And now I wonder if our lurking -- or resting -- friend can see the thermocline. It can be fun to watch as it roils and flows like the surface of two oceans meeting, one above and one below. Better than going crazy bored like a certain broody vampire.