What if Earth had two moons?

The consequences of a second moon orbiting the Earth depend on how massive that moon is and how far from the Earth it orbits. The most obvious effect would be that the ocean tides would be altered. Tides could be either smaller or higher and there could be more than two high tides per day. If the gravitational influence of a second moon were extreme, it could lead to phenomenally huge ocean tides (up to a kilometre high) which would also result in frequent tsunamis. It could also lead to enhanced volcanic activity and earthquakes.