Largest tetrapod troglodyte

The largest tetrapod troglodyte (cave-dweller) is the olm (Proteus anguinus), an aquatic salamander native to cave systems in Croatia, Italy and Bosnia-Herzegovina. It measures up to 40 cm (1 ft 3 in) long, and is very elongate, almost snake-like in body form, though it does possess two pairs of short limbs. Living entirely underground in subterranean rivers and pools, its functionally blind eyes have regressed beneath its skin, and its body is usually unpigmented, though the blood in its vessels gives its otherwise pallid skin a pink tinge.

In addition to being the world's largest tetrapod troglodyte, the olm is jointly the world's largest vertebrate troglodyte, sharing this latter record with Neolissochilus pnar, a recently discovered subterranean cyprinid fish from Meghalaya, north-east India, which also measures up to 40 cm (1 ft 3 in) long.

The olm lives a very inactive life, usually remaining within an area of just a few square metres throughout its life, with one studied specimen famously remaining entirely stationary for 2,569 days (over seven years!).

It is also very long-lived, with some zoo specimens having lived for more than 70 years, and with mathematical predictions of a maximum lifespan for this species exceeding 100 years. This also makes it the longest-lived amphibian currently known.

Known to science since 1689, olms are occasionally seen on the surface when washed there by heavy rains flooding their underground habitat, and several centuries when specimens were revealed in this manner they were thought by the local people to be the infants of some enormous cavern-dwelling dragon.