The calf muscle that lies beneath the gastrocnemius and helps plantar flex the foot is the soleus.
The soleus is a powerful, broad muscle in the superficial posterior compartment of the lower leg. It originates from parts of the tibia and fibula and joins the gastrocnemius through the calcaneal, or Achilles, tendon, which attaches to the heel bone. Unlike the gastrocnemius, the soleus crosses only the ankle, not the knee.
Plantar flexion is the movement that points the foot downward, as when rising onto the toes or pushing off during walking and running. The soleus is particularly important when the knee is bent, because bending the knee reduces the gastrocnemius's ability to generate force. It also contains many fatigue-resistant fibers and contributes to maintaining upright posture.
The most common mix-up is confusing the soleus with the visible gastrocnemius, which forms much of the calf's surface bulge. Both muscles plantar flex the foot and work together as the triceps surae, but the soleus sits deeper and is especially valuable for steady, sustained force.