The drive toward higher density and higher performance in integrated circuits creates a need to keep interconnects short and eliminate layers of packaging. In this article, we propose a novel, ultrahigh-density (exceeding 104 leads per cm2), compliant, wafer-level, input/output interconnection technology called “sea of leads” as a key enabling technology for future high-performance microsystems. The mechanical compliance is addressed through slippery leads (leads released from the surface) and embedded air gaps. The ability to fabricate embedded air gaps has enabled the integration of optical interconnects with high index-of-refraction mismatches between the core and cladding.