I have never liked Everett's many worlds hypothesis for both emotional and rational reasons, but the extra dimensions necessary for string theory? We can hope! Mini black holes, however, remain disturbing in spite of the assurances that they are too hot and unstable to last long enough to feed and become a threat.
I wonder if we could look for them in cosmic ray collisions with our upper atmosphere. The universe is a bigger and more powerful accelerator than any we will ever build, and it's already operating at no cost to us.