What is an ultramarathon, how far and on what terrain?

Ultra running is exploding in popularity around the world, but what actually constitutes ‘ultra’ in an ultramarathon?
First published in 1903, South China Morning Post is Hong Kong’s premier English language newspaper and has the city’s most affluent and influential readership. With a reputation for authoritative, influential and independent reporting on Hong Kong and China. The newspaper is supported with its online publication and its Sunday edition, Sunday Morning Post.
Ultra running is fast becoming a mainstream sport. Once, it was the realm of a few crazy runners, and not the pastime of everyone from your boss to your neighbour. But what actually is an ultramarathon?

Ultramarathon training: Everything you need to know

Margaret River Ultra Marathon - Margaret River Ultra Marathon

Koerner, Hal, Jurek, Scott, Chase, Adam W.: 0884768536342: Books

Hal Koerner's Field Guide to Ultrarunning: Training for an Ultramarathon, from 50K to 100 Miles and Beyond

How To Run Your First Ultra Marathon

Training for Your First Ultramarathon: How to Prepare With a Plan

Why run an ultramarathon? - FitPro Blog

Endurance-Based Workouts for Ultramarathon Training – iRunFar

Are You Too Big to Run an Ultra? - Runners Connect

Zach Bitter's Instagram, Twitter & Facebook on IDCrawl

Ultramarathon - Wikipedia