Chicagoan cornetist Rob Mazurek's notion of the underground has always been about freedom: freedom from oppression, from barriers and from limitation; and the freedom to adapt and mutate as and when it becomes necessary or desirable. Since 1996, he's led Chicago Underground Collective, a fluid arrangement ranging from the core duo with drummer Chad Taylor through various quartets and up to an orchestra. While he was resident in Brazil for five years from 2000, he initiated São Paulo Underground, a trio with local musicians that drew on samba and bossa nova as much as it did free-jazz, electronica and rock. This latest offering, recorded at London's Cafe OTO as part of the Underground's 20th anniversary celebrations, sees him drawing two of the UK's most active and mercurial improvisers into the fold for a brand new quartet. As you'd expect, the two British serial-collaborators fit right in, too, helping to create a dense, episodic experience that moves through moments of tense abstraction and brooding darkness to fierce, roiling tumult. With intelligent use of electronics, Mazurek's cornet is an animated presence throughout, unleashing dirty smears, fat blats and phosphorescent tracers – or just settling into a high, pure tone with more than a hint of Don Cherry. Roughly halfway into the set, Mazurek starts to chant the mantra: “Something must happen”. Sounds an awful lot like something just did.