Evidence is accumulating to show that the Earth's biosphere extends underground into deep igneous rock formations. In certain formulations, abiotic energy-yielding reactions between reduced rocks and groundwater provide a potential for in situ primary production by anaerobic microorganisms-- thus obviating any dependence on a surface ecosystem. Conceivably, such ecosystems could exist in the subsurface of other planets in the solar system. The main requirements are water, ferrous silicate minerals, carbon dioxide, and nitrogen. Unfortunately, observation of the subsurface is difficult. For example, current estimates suggest that the hydrosphere on Mars might be more than 2 km below the surface. Living SLMEs might be detected through conduits to the subsurface, such as wells, springs, or seeps in deep canyon walls. Signals produced by SLMEs might include cells, metabolic products (such as reduced gases) and their isotope ratios, and isotope ratios in residual substrates. Rocks in which now-defunct SLMEs once existed might be more accessible if they are brought to the surface by rock cycle processes. Signals of extinct SLME remnants have not yet been investigated, but might include microfossils, certain secondary mineralization patterns, and isotope ratios of secondary materials. Examples of both extant and extinct SLMEs have been identified on Earth, and are available for study and experimentation.