nether tunnels for transport

[frodo_gamgee]

if there is a tunnel in the nether, even if its a tunnel made by someone for their personal use. am i aloud to enter and use it for transport even without asking them first? or is this considered entering a private build without permission?

the person who built it hasnt been online since early last year, its just a side tunnel that leads farther into the nether than the public tunnel, so i use it the go out an iron door to get into open nether to get quartz ore and glowstone.