Understanding and predicting the formation of shell structure from nuclear forces is a central challenge for nuclear physics .
While the magic numbers N = 2 , 8 , 20 are generally well understood , N = 28 is the first standard magic number that is not reproduced in microscopic theories with two-nucleon forces .
In this Letter , we show that three-nucleon forces give rise to repulsive interactions between two valence neutrons that are key to explain ^ { 48 } Ca as a magic nucleus , with a high 2 ^ { + } excitation energy and a concentrated magnetic dipole transition strength .
Towards the neutron drip line , we predict a shell closure at N = 34 .
The repulsive three-nucleon mechanism improves the agreement with experimental binding energies and suggests the drip line around ^ { 60 } Ca .