Magnetic detection offers one approach to complement optical, infrared, radar, and acoustic sensing for wide-area detection, localization and classification applicable for a large class of subsurface structures. Sensors incorporating Superconducting Quantum Interference Devices (SQUIDs) provide a promising approach for such wide-area searches. A SQUID- based sensor developed in the 1980s demonstrated an effective capability to detect, localize, and classify underwater magnetic targets such as sea mines and unexploded ordnance. This sensor incorporated low critical temperature (low Tc) superconducting components cooled by liquid helium to a temperature of 4 degrees Kelvin (K). In the late 1980s, a new class of 'high Tc' superconductors was discovered with critical temperatures above the boiling point of liquid nitrogen (77 K). Nitrogen refrigeration offers new opportunities for this superconducting sensor technology, including significant reduction in the size of sensor packages and reduction in the requirements for cryogenic support and logistics. As a result of this breakthrough, a high-Tc sensor cryocooled by liquid nitrogen has been developed and is being evaluated for mobile operation. In this paper, opportunities and approaches using magnetic sensors for subsurface structure detection will be addressed. Results from evaluation of this high-Tc SQUID sensor will be presented and its capability for these applications will be discussed.