Terminology note: KS = Known Space, i.e. anywhere in null, low, or high-security space. JS = J-Space, i.e. anywhere inside a wormhole.

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To learn about a wormhole, right-click it and select Show Info. The top two sentences are irrelevant.

The third sentence (beginning, “This wormhole seems to lead…") narrows down the hole's destination:

• Plain "unknown" space is class 1-3 JS, “dangerous unknown" is C4-5, and “deadly unknown" is C6.
• High/Low/Null Security is that class of KS, "Triglavian" is Pochven and Thera is, you guessed it: Thera.
• If you're currently in KS you can further differentiate C1 from C2/3; if the last sentence says "up to medium ships" will fit, it's a C1.
• To suss C2 from C3 and C4 from C5 you must either yump in and look up the J-number or learn to discern holes visually.
  • Here's my old guide for that. Ashy has a more in-depth guide to visually IDing all wormholes, complete with KS region IDing.

The fourth sentence indicates a hole's remaining life:

• "This wormhole has not yet begun its natural cycle..." = max life ("fresh").
• "This wormhole is beginning to decay..." = between four hours and max life (also "fresh").
• "This wormhole is reaching the end of its natural lifetime." = 0-4 hours ("EOL" i.e. "end-of-life", “critical" in Tripwire).

The fifth indicates its remaining mass:

• "This wormhole has not yet had its stability reduced..." = over 50% of mass remaining ("stable" or "healthy").
• "This wormhole has had its stability reduced..." = 10-50% of mass remaining (“destab", "shrunk" or “first shrink").
• "This wormhole has had its stability critically disrupted..." = under 10% of mass remaining ("crit").

The last sentence indicates what size class the hole is, i.e. which types of ships will fit through.

• "Only the smallest..." = S: destroyers, ZMPE'd HICs and smaller (these are often called “frigholes", somewhat confusingly).
• "Up to medium..." = M: battlecruisers, industrials, Nestors with no onlined plates and smaller (no other battleships).
• "Larger ships..." = L: all other battleships, Orcas, and smaller (one type of L—Pochven's C729 reverse-static—can also fit Bowheads).
• "Very large..." = VL: all ships up to and including freighters/JFs/Rorqs or all ships up to and including carrier/dread/FAX (the latter subgroup includes the former).

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WARNING: NOT ALL HOLES OF A PARTICULAR SIZE CLASS WILL ALLOW THE SAME NUMBER OF SHIPS THROUGH.
     That is to say: only S holes all have the same total mass limit. M, L and VL vary within their own classes.
     For example, D845—an L—will pass 5 million tonnes, whereas Unidentified Wormholes—also Ls—only pass 750,000 tonnes.

The jump range of wormholes is 5 km (double that of stargates/Ansiblexes).

To see how much mass a particular hole has, you must identify its type, which is the non-K162 side. Then you can Control + F on this page.
     You can also use the above link to check whether a VL is freighter-and-smaller or caps-and-smaller (but don't be dumb with either).

All frigholes are now effectively unrollable. They were rollable for years due to a bug and many evictors and risk-averse krabs cried like stuck pigs when they were fixed.
     I think it's great that they're unrollable. The risk should be factored into eviction planning and krabs should HTFU. YMMV. May Bob bless and keep you.

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Unidentified or "Uni" or "Drifter" wormholes have different mechanics to normal holes and are only used for certain purposes. A Uni TLDR:

• Both the KS and JS sides of Uni-to-KS connections have deadspaced warpins 89 km from their holes.
  • You can land on these holes at zero if you're lucky or have prep time.
  • These holes can be rolled like normal holes (with additional risks).
• The KS sides:
  • have entrances which spawn only in systems with Jove Observatories;
  • if present, show on the overview as a beacon called Unidentified Wormhole (with no corresponding scannable signature);
  • lead to one of five JS systems colloquially named after unique combat sites inside them which are indicated by the first letter of the entrance wormhole:
    • B735 = Barbican = J110145,
    • C414 = Conflux = J200727,
    • R259 = Redoubt = J174618,
    • S877 = Sentinel = J055520,
    • V928 = Vidette = J164710;
  • and are randomly patrolled by these three classes of NPCs, none of which can be easily handled if unprepared:
    • Drifters—i.e. [Name] Tyrannos—which have very high tank and DPS and a doomsday that can insta-zap subs (and dumb caps),
    • Diamond rats—only in the regions where they naturally spawn—which will shred unwitting fleets, and
    • Autosythian Lancers, which can be tanked en route to a hole w/logi but which can easily pop unsupported subs.
• The JS sides have no convenient wormhole beacons. Instead, there are many scannable signatures, all of which are wormholes.
  • The majority are KS-bound deadspaced holes, but there's always at least one connection to other JS; such holes aren't deadspaced.
• The holes have, on average, 750,000 tonnes worth of transit capacity and fit BS/Orcas and smaller.