WILLISTON, N.D. (AP) — Three massive fires since the beginning of June have highlighted the threat lightning poses in the North Dakota oil patch, and in each case it was tanks that store the toxic saltwater associated with drilling — not the oil wells or drilling rigs — that were to blame.

The lightning-sparked fires destroyed the groups of silo-like storage tanks at the three locations, which are among more than 440 sites in North Dakota where so-called saltwater is stored before being pumped into permanent disposal sites miles underground. In each case, the fires burned for days, spewing noxious black smoke into the air and literally salting the earth.