A big earthquake occurred south of Istanbul in the summer of 2016, but it was so slow that nobody noticed. The earthquake, which took place at mid-crustal depth, lasted more than 50 days. A novel processing technique applied to data from special borehole strainmeter instruments and developed by researchers from the GFZ German Research Centre for Geosciences and collaborators successfully identified the ultra-slow quake below the Sea of Marmara. The team, led by Patricia Martínez-Garzón, has published the results in Earth and Planetary Science Letters.