Detection of gravitational waves (GWs) from binary black holes (BHs) by Advanced LIGO has opened a new window of astronomical observation. Many conceivable sources such as intermediate-mass BH binaries and white dwarf binaries, as well as stellar-mass BH inspiral, would emit GWs below 10 Hz. It is highly desirable to open a new window in the infrasound frequency band below 10 Hz. A low-frequency tensor detector could be constructed by combining six magnetically levitated superconducting test masses. Seismic noise and Newtonian gravity noise are serious obstacles in constructing terrestrial GW detectors at low frequencies. A tensor detector can reject the near-field Newtonian gravity noise more efficiently. Such a detector is equally sensitive to GWs coming from anywhere in the sky, and is capable of resolving the source direction and wave polarization. I will present a design concept of a new low-frequency detector, named SOGRO, which could reach a strain sensitivity of 10-19-10-21 Hz-1/2 at 0.1-10 Hz. I will discuss ways to mitigate the seismic and Newtonian noise, as well as foreseeable technical challenges and limitations in developing such a detector.