  Water ice is thought to be trapped in large permanently shadowed regions
(PSRs) in the Moon's polar regions, due to their extremely low temperatures.
Here, we show that many unmapped cold traps exist on small spatial scales,
substantially augmenting the areas where ice may accumulate. Using theoretical
models and data from the Lunar Reconnaissance Orbiter, we estimate the
contribution of shadows on scales from 1 km down to 1 cm, the smallest distance
over which we find cold-trapping to be effective for water ice. Approximately
10-20\% of the permanent cold trap area for water is found to be contained in
these "micro cold traps," which are the most numerous cold traps on the Moon.
Consideration of all spatial scales therefore substantially increases the
number of cold traps over previous estimates, for a total area of ~40,000 km^2.
A majority of cold traps for water ice is found at latitudes >80{\deg} because
permanent shadows equatorward of 80{\deg} are typically too warm to support ice
accumulation. Our results show that water trapped at the lunar poles may be
more accessible as a resource for future missions than previously thought.
