Measurements of the motions of disk-like intruders in rectangular containers filled with a variety of granular materials reveal that the time required for the intruder to reach the surface from an initial position at the bottom of the container depends on the ratio of the intruder's density to
that of the background granules. The rise time is longest for intruders with densities between about 0.4 and 0.6 times the density of the granular material, and becomes shorter for intruders that are much lighter or much heavier than these limits.