The Tcha'besheh, referred to as the A'kweth or Underlier, was a large silicon based life form that lived on the planet Vulcan.

Very little is known about the creatures biology or their evolution. They do not apparently need to respire, require oxygen or feed and were one of the first recorded species that lived on a planet alongside a carbon based lifeform; namely the Vulcans. This would change somewhat upon the discovery of the Horta's of Janus VI. They were large beasts that were the size of a great house and possessed several tentacles.

Some have compared the creatures to the Dinosaurs of the planet Earth, however unlike the large reptiles of the Human homeworld, the Tcha'beseh never went extinct and continued a secluded life far beneath Vulcan's surface. Scans were difficult if not impossible to get from the creatures as the natural elements between the equipment would prevent an individual from getting many readings. What is gained from such scans include a life-sign reading that would suggest a level of vitality equal to a thousand creatures which make pinpointing an Underlier very difficult.

They are considered to be the Vulcan equivalent of Whales that travel beneath the sands of their homeworld.

In the ancient days when the proto-Vulcans developed tribes but not speech, one such Vulcan known as The Wanderer learnt his first word through an encounter with a Tcha'beseh. This would be shortly before an intense solar flare from 40 Eri would transform Vulcan into a desert world. (TOS novel: Spock's World)