1.   A huge underground salt formation, where volatile hydrocarbons are stored, is the crux of the problem.

2.   Derived from an underground formation from which water flows naturally to the surface of the earth.

3.   Pipeline companies might have technology to look through rocks at underground formations.

4.   Seismic data is collected by vibrating the ground with sound, then using sensors to collect the echos that bounce back from underground geological formations.

5.   The radioactive fluids went directly into underground formations -- not into cement mixes or steel liners.

6.   The systems are used to collect data for three-dimensional images of underground formations, which help in detecting oil and gas fields.

7.   To generate the data, ships bounce sound signals off underground formations.

8.   Groundwater is contained in underground geologic formations called aquifers, which are constantly replenished by rainwater that percolates downward through the soil.

9.   Nearby excavations revealed underground stone formations.

10.   Unusually large amounts of water flowing into porous underground formations can make the land swell up like a flat sponge that suddenly grows when tossed into water.