Airships

Airships are large-scale aircrafts used for travel via flight.

Description
Marulk's father used an airship to travel to and into the Abyss which resulted in a fatal crash costing him and his crew their lives; with only Marulk surviving.

The discovery of the Abyss during Chapter 1 of the Manga shows that a primitive version of this technology might've already existed 1900 years prior to the beginning of the series.