A plurality of gas sample collecting containers are distributed in a predetermined pattern from a vehicle. The containers are cup shaped but weighted so that the open end sinks first and engages the subaqueous earth laer. Gas migrating upwardly through the earth is caught by the inverted cup. The gas sample may be either collected in a separate container or exposed to a detector material located above the expected water level. The sample collecting containers may be retrieved by means of marker floats, magnetic grappling devices or self actuated flotation devices.