Back 2 Base-ix have released several new types of inverted bases.
From their announcement:
Tired of trying to trim your bases to add depth detail? And then adding a layer to the bottom to hold all your pieces together while they set?
Well, it just got a whole lot easier.
Inverted bases have their flat surface at the bottom of the base (surface playing level). This allows for ‘depth’ detail to be added, all the while being supported by the base itself. Great for holding water effects, or adding ‘underground’ details before mounting your models.