In this paper we show how to explicitly write down equations of hyperelliptic curves over Q such that for all odd primes ℓ the image of the mod ℓ Galois representation is the general symplectic group. The proof relies on understanding the action of inertia groups on the ℓ-torsion of the Jacobian, including at primes where the Jacobian has non-semistable reduction. We also give a framework for systematically dealing with primitivity of symplectic mod ℓ Galois representations.The main result of the paper is the following. Suppose n = 2g +2 is an even integer that can be written as a sum of two primes in two different ways, with none of the primes being the largest primes less than n (this hypothesis appears to hold for all g = 0, 1, 2, 3, 4, 5, 7 and 13). Then there is an explicit N ∈ Z and an explicit monic polynomial f 0 (x) ∈ Z[x] of degree n, such that the Jacobian J of every curve of the form y 2 = f (x) has Gal(Q(J[ℓ])/Q) ∼ = GSp 2g (F ℓ ) for all odd primes ℓ and Gal(Q(is monic with f (x) ≡ f 0 (x) mod N and with no roots of multiplicity greater than 2 in Fp for any p ∤ N .