If it's that important, you shouldn't bank on one medium only. I'd suggest also burning DVDs on archival quality media (Verbatim Datalife, etc.), and having multiple copies on each type of media. Temperature also plays a part. You may want to look into having it stored not underground but in humidity/temperature controlled facilities that are also earthquake resistant. Large companies usually send their backup tapes on a regular basis to such facilities. Back in the days when I was in charge of stuff like this, the best company was Arcus; I think they have since been bought out by Iron Mountain.