Let F q be a finite field of odd characteristic and K = F q (t). For any integer d ≥ 2 coprime to q, consider the elliptic curve E d over K defined by y 2 = x · x 2 + t 2d · x − 4t 2d . We show that the rank of the Mordell-Weil group E d (K) is unbounded as d varies. The curve E d satisfies the BSD conjecture, so that its rank equals the order of vanishing of its L-function at the central point. We provide an explicit expression for the L-function of E d , and use it to study this order of vanishing in terms of d.Of course, the second assertion implies the first one, and our treatment might appear redundant. However, the two proofs shed different lights on the behaviour of the sequence d → rank E d (K) . Indeed, for the explicitly constructed (d n ) n≥1 in Theorem 6.4, the corresponding curves E dn have 'very large' ranks (as large as allowed by Brumer's bound, cf.[Bru92] and our Remark 6.6(b)). However, these integers d n are very far apart from each other, and they cannot be deemed representative of the 'typical' size of rank E d (K) . The average result fills in that gap by showing that the rank of E d is 'usually large'; but one then loses the explicit and precise character of the first construction.The second proof also reveals that the sequence {E d } d at hand is quite special: indeed, Brumer has proved that the average rank of elliptic curves over F q (t) is bounded (see [Bru92]).Let us now explain the strategy of the proof of Theorem A as we give the plan of the paper. In section 1, we start by introducing the elliptic curves E d and by computing their relevant invariants. We then describe the torsion subgroup E d (K) tors (Theorem 1.6), and provide a point of infinite order in E d (K) (Corollary 1.7). We also explain why the results in [Ulm07] cannot be used here.Our first step towards the proof of Theorem A will be to give an explicit formula for the L-function of E d /K. To avoid introducing too many notations here, let us only state the following special case of our result (see Theorem 3.1 for the general version):where, for all n ∈ {1, . . . , 2d − 1} with n = d/2, 3d/2, we letThe relevant objects are introduced in section 2 and the proof of Theorem 3.1 is given in section 3. It is based on direct manipulations of character sums related to 'counting points' on the various reductions of E d . Given that there are very few elliptic curves over K for which the L-function is explicitly known, this Theorem may be of independent interest. The expression of L(E d /K, T ) in Theorem B is sufficiently explicit that one can study its order of vanishing at T = q −1 (see Corollary 3.3). Our main interest in doing so is that the BSD conjecture is known to hold for E d /K i.e., one has:This fact has been proved by Berger in [Ber08], and we sketch her proof in section 4. To this effect, we also briefly recall there how the curves E d are constructed.The expression for ord T =q −1 L(E d /K, T ) obtained in Corollary 3.3 becomes more tractable for certain values of d: specifically, for those d ≥ 1 such that 2d is su...