eater.net clock: messy negative edge

Just unpacked this oscilloscope and I thought I'd measure the signal from the eater.net 8-bit computer clock module. The rising edge jumping from 0v to 5v looks fine, but the trailing edge looks terrible: it's diving to -5v, jumping back to to +5v, then there's a damping oscillation to 0. The whole transition takes about 45 ms.

Zoomed in on the transition from 5v to 0v (vert 1v/div, horiz 5ms/div)