We present a scanning probe microscope based on optical tweezers for three-dimensional imaging
of the topology of transparent material in the nanometer range. A spherical nanoparticle serves as a
probe. An optical trap moves it through the sample ~e.g., a polymer network!, while the position of
the particle center is recorded by three-dimensional interferometry. Accessible volumes are
reconstructed from the histogram of thermal position fluctuations of the particle. The resolution in
determining the position of surfaces in three dimensions is about 20 nm.