  The earliest phases of massive star formation are found in cold and dense
infrared dark clouds (IRDCs). Since the detection method of IRDCs is very
sensitive to the local properties of the background emission, we present here
an alternative method to search for high column density in the Galactic plane
by using infrared extinction maps. We find clouds between 1 and 5 kpc, of which
many were missed by previous surveys. By studying the physical conditions of a
subsample of these clouds, we aim at a better understanding of the initial
conditions of massive star formation. We made extinction maps of the Galactic
plane based on the 3.6-4.5 microns color excess between the two shortest
wavelength Spitzer IRAC bands, reaching to visual extinctions of ~100 mag and
column densities of 9x10^22 cm^-2. From this we compiled a new sample of cold
and compact high extinction clouds. We used the MAMBO array at the IRAM 30m
telescope to study the morphology, masses and densities of the clouds and the
dense clumps within them. The latter were followed up by pointed ammonia
observations with the 100m Effelsberg telescope, to determine rotational
temperatures and kinematic distances. Extinction maps of the Galactic plane
trace large scale structures such as the spiral arms. The 1.2 mm emission maps
reveal that the high extinction clouds contain extended cold dust emission,
from filamentary structures to still diffuse clouds. Most of the clouds are
dark in 24 microns, but several show already signs of star formation via maser
emission or bright infrared sources, suggesting that the high extinction clouds
contain a variety of evolutionary stages. The observations suggest an
evolutionary scheme from dark, cold and diffuse clouds, to clouds with a
stronger 1.2 mm peak and to finally clouds with many strong 1.2 mm peaks, which
are also warmer, more turbulent and already have some star formation signposts.
