  Form a random k-SAT formula on n variables by selecting uniformly and
independently m=rn clauses out of all 2^k (n choose k) possible k-clauses. The
Satisfiability Threshold Conjecture asserts that for each k there exists a
constant r_k such that, as n tends to infinity, the probability that the
formula is satisfiable tends to 1 if r < r_k and to 0 if r > r_k. It has long
been known that 2^k / k < r_k < 2^k. We prove that r_k > 2^{k-1} \ln 2 - d_k,
where d_k \to (1+\ln 2)/2. Our proof also allows a blurry glimpse of the
``geometry'' of the set of satisfying truth assignments, and a nearly exact
location of the threshold for Not-All-Equal (NAE) k-SAT.
