Mechanically active single-crystal structures can now be fabricated with size scales of less than 100 nm in all three dimensions. These engineered structures have fundamental mechanical resonance frequencies approaching 1 GHz, with very little mechanical dissipation. Using such structures, we are developing a new class of devices with applications in electrometry, magnetometry, surface physics, and investigations of the physics of phononmediated processes. The methods used to fabricate these devices, combining electron-beam
lithography and high precision etching, have been applied mostly to single-crystal Si and IIIV substrates, but extension to other material systems is being pursued. In addition to practical
applications, there exists the possibility of investigating fundamental quantum mechanical processes such as macroscopic quantum tunneling, in a mechanical system. In this report we describe our approaches to this topic.