Discovery magazine has an article revisiting a disputed claim from five years ago about recovery of living bacteria from 250 million year old salt crystals. Russel Vreeland and colleagues had removed salt crystals from a Permian rock layer from a mine about 500 meters underground; sterilized the outsides; and used drilled cores from these to inoculate growth medium. On three occasions the cores succesfully cultured bacteria related to modern salt-tolerant species- a literal sleeping beauty.
The initial work received a lot of attention but was not universally accepted. There were two concerns. The first one, which they tried heroically to address, is that the isolates were modern contaminants. I still don't think this is closed. The second is whether the salt crystals really belonged in the rock formation from which they were obtained. New salts can force their way into old rock layers.
The new work used two methods to judge if the salts belong in the aged formation. The first test was that the ion composition of the salts is consistent with a brine formed in a shallow sea (and not a modern mineshaft). The second is that gasses, trapped in fluid inclusions within the salt crystals, had been trapped between 60 and 80 degrees Farenheit- again consistent with formation in a seashore, not a mineshaft. (The method used, cooling nucleation--brand new to me-- is discussed here. ) So the salts are at least chemically consistent with belonging to the Permian rocks around them.
My inclination is to accept that bacteria could sporulate for a very long time, especially in high salt. I would be more comfortable with these particular isolates if the DNA were to be a bit more different than modern forms. (They sequenced the DNA for 16S RNA and got a 92% hit to a modern Bacillus).