  We study the clustering of matter in the intergalactic medium from the
Lyman-alpha forests seen in the spectra of pairs or groups of z ~ 2 quasars
observed with FORS2 and UVES at the VLT-UT2 Kueyen ESO telescope. The sample
consists of five pairs with separations 0.6, 1, 2.1, 2.6 and 4.4 arcmin and a
group of four quasars with separations from 2 up to 10 arcmin. This
unprecedented data set allows us to measure the transverse flux correlation
function for a range of angular scales. Correlations are clearly detectable at
separations smaller than 3 arcmin. The shape and correlation length of the
transverse correlation function on these scales is in good agreement with those
expected from absorption by the photoionized warm intergalactic medium
associated with the filamentary and sheet-like structures predicted in CDM-like
models for structure formation. At larger separation no significant correlation
is detected. Assuming that the absorbing structures are randomly orientated
with respect to the line of sight, the comparison of transverse and
longitudinal correlation lengths constrains the cosmological parameters (as a
modified version of the Alcock & Paczy\'nski test). The present sample is too
small to get significant constraints. Using N-body simulations, we investigate
the possibility to constrain Ol from future larger samples of QSO pairs with
similar separations. The observation of a sample of 30 pairs at 2, 4.5 and 7.5
arcmin should constrain the value of Ol at +- 15 % (2 sigma level). We further
use the observed spectra of the group of four quasars, to search for underdense
regions in the intergalactic medium. We find a quasi-spherical structure of
reduced absorption with radius 12.5 h^{-1} Mpc which we identify as an
underdense region.
