The most extreme star formation in the local Universe is found in the deeply embedded star clusters of starburst dwarfs; they hold up to 10^6M⊙ of stars younger than 5 Myr in a few pc^3. What kind of clouds form these clusters, and what do the clusters do to their clouds? With ALMA, the SMA and TEXES we measure molecules, ions and dust with high spatial and spectral resolution. We have found infalling molecular streams, outflowing ions, and narrow interaction zones between them. The star formation efficiency is very high and the embedding clouds are hot and dusty.