In ΛCDM, the BAO scale is treated as a nearly rigid comoving standard ruler, so after accounting for known nonlinear effects and reconstruction, its measured position should not depend strongly on whether galaxy pairs lie in overdense superclusters or underdense voids (Eisenstein et al. 2007; Seo & Eisenstein 2007). Yet density-split and environment-split analyses find that the BAO peak appears compressed in overdense regions and stretched in underdense ones at the percent level or more, challenging the assumption of a strictly environment-independent ruler and raising questions about whether standard modeling of nonlinear evolution, bias, and reconstruction is fully adequate (Roukema et al. 2015; Neyrinck et al. 2018).