Tucked between the Thoothukudi and Tirunelveli districts of Tamil Nadu lies Theri Kaadu, a striking red-sand landscape often likened to the surface of Mars. Spanning about 500 square kilometers, this “red desert” earns its colour from high concentrations of iron oxide in the sand.

Geologically speaking, Theri Kaadu is a product of aeolian (wind-driven) deposition and ancient marine sediment. Roughly 10,000 years ago, during periods when sea levels were lower, exposed seabeds and sediments, plus wind from the exposed marine shelves and the Western Ghats, transported iron-rich sand inland where it eventually accumulated as dunes.

Unlike classical deserts formed by extreme dryness, Theri Kaadu’s “desert-like” state arises more from shifting dunes over ferruginous soil than an absence of moisture altogether. The red dunes are categorised into three types based on proximity to the coast: inland theris (more stable), coastal theris (highly active), and theris close to the coastline with moderate activity.

Flora and fauna here are sparse but resilient—species such as fan-throated lizards, spotted owlets, and velvet insects (velvet poochi) survive in this harsh, red expanse. Shrubby vegetation, palms, some trees like cashew are found, though survival is tough for most plants.

Theri Kaadu isn’t just scenic—it’s a living record of Earth’s changing climate, sea levels, and wind patterns. Its red dunes whisper tales of marine pasts and shifting sands, making it a natural wonder and a geological classroom for those curious about Earth’s deep history.