We explore theoretically the notion that the underdoped cuprates may be viewed as doped U(1) spin liquid Mott insulators. We pursue a conceptually clear version of this idea that naturally incorporates several aspects of the phenomenology of the cuprates. We argue that the low-doping region may be fruitfully discussed in terms of the universal physics associated with a chemical-potential-tuned Mott transition between a U(1) spin liquid insulator and a d -wave superconductor. A precise characterization of the deconfinement in the U(1) spin liquid is provided by the emergence of a conserved gauge flux. This extra conservation law should hold at least approximately in the underdoped materials. Experiments that could possibly detect this conserved gauge flux are proposed.