There appears to have been some confusion regarding some details on the engine and the project itself, so this post's more for answering some recent questions than an update on the progress of development (which, in all honesty remains mostly stagnant on the account that Geobox itself has not been distributed for use).
Affiliation with Geobox:
Geobox was not created to be used in Beneath the Surface. In fact, Geobox's original development predates the conceptualization stage of BTS by over a year.
Our project just so happens to use Geobox as an engine, but the connection should not be stretched beyond that.
Geobox is not "the Beneath the Surface engine", it's its own thing, used in other projects and games aside from this.
Any details regarding plot information are not concrete or final, as the script goes through reworks frequently. As such, locations, plot elements and even characters are not necessarily represented in the way they may appear in-game.
At the current point in time an actual demo release seems like a distant possibility, an unforunate contrast to how hopeful it seemed at multiple points thoughout the development history. That was of course, before we ended up switching engines.
The closest thing you're getting to a tech demo is the old single room-area with the GM engine, and given permission, a possible release of the Geobox tech demo, which has been showcased in small video incriments on the Cave Story Tribute Site Forums.
(I can't stress this enough, the Geobox tech demo has nothing at all to do with BTS)
Updating on this blog's become increasingly difficult, considering every member of the development team has their own personal priorities, in most cases, education or a job, and as a result, we end up with less of anything substantial to show here.
I apologize for how uneventful this all is.