Sunken Trampoline - worth a read before installing, love the grass covering
A family friendly backyard! (decorology)
A family friendly backyard! More
Safe and Cool: A Sunken Trampoline For Kids
The nitty gritty about building your own sunken trampoline.
Crazily detailed instructions on how to build a sunken trampoline.
partially sunken trampoline.... I would consider this. this might be good. I don't like the fully sunken ones though because you have to have such a deep hole, and you need vents for the air to escape or else it won't bounce.
DIY Inground Trampoline Instructions. We'll use the cinderblock idea to keep it level.