I have got the hang of basic construction, terrain modification, applying materials, placing trees, etc. I have created a water plane, positioned my terrain above it, and am able to sink lakes and pools, and raise hills and mountains. Built a couple of primitive structures too.
Initially (before mucking about with the Y position of the terrain) I was also able to invoke Gameplay mode and walk around my game using FPS Character Controller. I could walk into and out of my structures and hike over the hills and valleys of my terrain.
However, since changing the Terrain Heightmap to make the "sinkhole" (negative altitudes) work, I am having the strangest trouble with my FPS Character Controller. I position it at a reasonable height above the terrain, and everything looks fine in Editor mode; but when I start Play mode my FPS character starts out miles below the world, looking up at the underside of the terrain.
So far nothing I have tried will make the FPS Controller start in the location where I have dragged it in Editor mode. I have tried Saving Scene and exiting and reloading. I have deleted and recreated the FPS controller. No matter what I do, it persists in initializing itself in this oddball "way below the world in outer space" location.
I have googled diligently but have not found any mention of this problem. Presumably I'm skipping some important step, but tutorial videos don't offer any advice beyond "drag the fps controller into the scene and it should work."