Groundwater is fresh water located in the subsurface pore space of soil and rocks. They can also occur from the overpumping and extraction of groundwater and subsurface fluids. It includes the sediment surface and some subsurface layers. Seismic monitoring is a type of indirect subsurface monitoring. Water flows out of wetlands by evapotranspiration, surface runoff, and subsurface water outflow. Vapor intrusion can occur from sites with extensive subsurface solvent contamination. These areas may be wetlands fed by subsurface ethane and methane springs. Methanogenic microbial life forms in the subsurface are among possible sources. Stability appears to be conducive to trapping and retaining hydrocarbons under the subsurface. Infiltration to recharge subsurface storage increases baseflow. Water that seeps out of abandoned subsurface mines can also be highly acidic. Slight deviations are caused by inhomogeneities of subsurface structure. This can be delivered from surface, subsurface or air units. Oxidation and reduction affects the migration of arsenic in subsurface environments. Large amounts of water ice are believed to be present in the Martian subsurface.