Environmental Sealing is a series of filters, air tanks, hermetic seals, and scrubbers that allow a unit to operate in hostile environments like the vacuum of space, under water, or toxic waste dumps. All BattleMechs, aerospace and conventional fighters, DropShips, and submarine vehicles include environmental sealing as part of their construction. This sealing takes up no additional weight or space as it's built into the unit. Other units, like IndustrialMechs and combat vehicles, can be equipped with environmental sealing at a cost of 10% of the unit's weight. Support vehicles may incorporate environmental sealing into their design process (see TM p. 133).  Combat vehicles don't need to allocate any additional space for the sealing. However, IndustrialMechs need to allocate additional room in their chassis for the environmental sealing systems.
Any unit with Environmental Sealing is able to negotiate shallow water (Depth 1), but to operate in space or underwater effectively a unit needs to have a fusion, fission, or fuel cell engine. (An ICE requires oxygen to operate, which is obviously not available underwater.)
- TechManual, p. 216, "Environmental Sealing"
- Tactical Operations, p. 302, "Environmental (Vacuum) Sealing"
- Jihad Turning Points: Atreus, p. 17, "PM6 Peacemaker Recordsheet"
- Technical Readout: Vehicle Annex, p. 202, "SC-V-1 Vampyr ScavengerMech"