1. INTRODUCTION
2. THE SAMPLE OF EXTREME, CANDIDATE HIGHLY OBSCURED AGNS FROM THE NUSTAR SERENDIPITOUS SURVEY
2.1. A note on associated sources
3. DATA
3.1. Soft X-ray counterparts to the extreme NuSTAR sources
4. X-RAY PROPERTIES
4.1. X-ray spectral modeling
4.2. Results for the X-ray source properties
5. INDIRECT ABSORPTION DIAGNOSTICS
6. OPTICAL PROPERTIES
6.1. Optical spectra
6.2. Host galaxies
7. THE PREVALENCE OF COMPTON-THICK ABSORPTION
8. SUMMARY