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Oct 1, 2011 at 1:30 vote accept Richard Montgomery
Sep 26, 2011 at 4:00 vote accept Richard Montgomery
Oct 1, 2011 at 1:30
Sep 26, 2011 at 3:56 comment added Richard Montgomery Yes! That is our base! We, Rick Moeckel and I, arrive at it by starting with the the planar 4 body problem of celestial mechanics. Quotient by translations and rotations. Levi-Civita'' regularize the binary collisions. Result:the phase space is the cotangent bundle of the cone of Hirzebruch's surface BEFORE regularizing the 4 points. Do the same for 3 bodies. Result: the cotangent bundle of the cone over Noam's $a^2 + b^2 + c^2 =0$ conic. Fibration: forget one of the bodies''.
Sep 25, 2011 at 19:52 comment added Noam D. Elkies ...and the base is then the conic $a^2+b^2+c^2=0$, with the octahedral symmetry given by the signed permutation group, and the six vertices of the octahedron at $abc=0$.
Sep 25, 2011 at 16:48 history edited Noam D. Elkies CC BY-SA 3.0
Added identification with universal curve over X(4), likely answering OP's first question "do you know this second K3?"
Sep 25, 2011 at 3:13 history answered Noam D. Elkies CC BY-SA 3.0