We show that two distinct level sets of the vorticity of a solution to the 2D Euler equations on a disc can approach each other along a curve at an arbitrarily large exponential rate.on D × (0, ∞), with initial data ω(·, 0) = ω 0 .(1.2) We will consider the case when the vorticity ω = −∇ × u (which will be more convenient for us than the more standard ω = ∇ × u) is bounded, that is, ω 0 ∈ L ∞ (D). The customary no-flow boundary condition u · n = 0 on ∂D × (0, ∞), with n the unit outer normal vector, then yields the Biot-Savart lawIt has been known since the works of Hölder [4] and Wolibner [11] that solutions to the Euler equations on smooth two-dimensional domains remain globally regular, and that ∇ω(·, t) L ∞ cannot grow faster than double-exponentially as t → ∞ (although this bound seems to have first explicitly appeared in [12]). That is, for eachCt for each t ≥ 0. Whether the double-exponential rate of growth is attainable had been a long-standing open problem. The first examples of smooth solutions for which the vorticity gradient grows without bound as t → ∞ were constructed by Yudovich [13, 14]. Later Nadirashvili [10] and Denisov [1] provided examples with at least linear and superlinear growth, respectively. The period of relatively slow progress in this direction was ended by a striking recent result of 1