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MATERIALS AND METHODS Site of sampling. To have access to deep-subsurface samples, we used the Underground Research Facility, a mine gallery located at a depth of 224 m in the Boom clay formation near Mol, Belgium. The geological cross section is as follows: 0 to 188 m deep, Neogene sands; 188 to 280 m, Boom clay; and 280 to 450 m, alternating sand and clay formations. The Boom clay, dating from the Rupelian period (30 to 35 million years ago), is dominated by illite-smectite of marine origin; 50 to 60% of the material is smaller than 2 m in diameter, and 40 to 45% is between 2 and 60 m; water content is ϳ20% (wet weight); the median porosity is between 10 and 20 nm, and the largest pore size does not exceed 0.1 to 0.2 m in diameter (11a); total organic content is ϳ3% (dry weight); the pH is 8.2 to 8.8; and the sediment temperature is approximately 20ЊC. The interstitial clay water is more than ϳ35,000 years old, consistent with its very low hydraulic conductivity (, ϳ10 Ϫ12 m s Ϫ1) (5). Sampling conditions. Clay samples have been collected at 0

The cultural bacterial strains of two sediment samples, i.e., 260 strains, were submitted to numerical taxonomy to determine "ecological profiles." From these profiles several calculations of bacterial diversity were done with increasing number of strains (between 10 and 130). Studying 20-30 strains was sufficient to obtain a diversity of bacterial community.Number of tests could be reduced from 62 to 30 without any influence on bacterial diversity. Similarity between studied tests was shown by using numerical taxonomy.