Bathtub Effect

Underground storage tanks set in less porous, less absorbent, soils are susceptible to rising groundwater.  Those groundwaters may not recede in a normal fashion.

Accumulated groundwater can damage the bottom of a tank or even displace it by seeking to float.

This is called the Bathtub Effect where groundwater holds in place instead of draining. This is why you need an automatic tank gauge to prevent this effect from happening.