MEGAN ("MEtaGenome ANalyzer") is a computer program that allows optimized analysis of large metagenomic datasets.
Metagenomics is the analysis of the genomic sequences from a usually uncultured environmental sample. A large term goal of most metagenomics is to inventory and measure the extent and the role of microbial biodiversity in the ecosystem due to discoveries that the diversity of microbial organisms and viral agents in the environment is far greater than previously estimated.  Tools that allow the investigation of very large data sets from environmental samples using shotgun sequencing techniques in particular, such as MEGAN, are designed to sample and investigate the unknown biodiversity of environmental samples where more precise techniques with smaller, better known samples, cannot be used.
Fragments of DNA from an metagenomics sample, such as ocean waters or soil, are compared against databases of known DNA sequences using BLAST or another sequence comparison tool to assemble the segments into discrete comparable sequences.  MEGAN is then used to compare the resulting sequences with gene sequences from GenBank in NCBI.  The program was used to investigate the DNA of a mammoth recovered from the Siberian permafrost and Sargasso Sea data set.