  We give an explanation for the signal detected by NANOGrav as the stochastic
gravitational wave background from binary mergers of primordial "Stupendously
Large Black Holes" (SLABs) of mass $M\sim(10^{11}-10^{12})M_{\odot}$, and
corresponding to roughly $0.1\%$ of the dark matter. We show that the stringent
bounds coming from $\mu$ distortions of the CMB can be surpassed if the
perturbations resulting in these BHs arise from the non-Gaussian distribution
of fluctuations expected in single field models of inflation generating a spike
in the power spectrum. While the tail of the stochastic background coming from
binaries with $M\lesssim 10^{11}M_{\odot}$ could both fit NANOGrav and respect
$\mu$ distortions limits, they become excluded from large scale structure
constraints.
